Swiss meringue buttercream is a Swiss-style frosting made by gently heating egg whites and sugar, whipping them into a glossy meringue, then beating in butter. It is popular on cakes and cupcakes for its smooth texture and balanced sweetness; versions vary, but this method makes a frosting that pipes neatly and spreads evenly.
Ingredients
- 5 large egg whites, about ⅔ cup (150 g), ideally at room temperature
- 1 cup (200 g) caster sugar
- 1½ cups (340 g) unsalted butter, softened but still cool, cut into small cubes
- 2 tsp (10 ml) vanilla extract
- ¼ tsp (1 g) fine salt
Use fresh shell eggs and separate them carefully: even a trace of yolk or grease can make the whites harder to whip. Caster sugar is the fine white sugar commonly sold in the UK; if you only have granulated sugar, it can work, but the larger crystals may take longer to dissolve. The butter should give slightly when pressed without looking shiny or greasy. This quantity makes about 5 cups of frosting, enough to generously cover and fill a two-layer 20 cm (8-inch) cake or to pipe 24 cupcakes.
Instructions
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Prepare the equipment. Put a heatproof mixing bowl over a saucepan with a few centimetres of gently simmering water. The base of the bowl must sit above the water rather than touch it. Wash and dry the bowl and whisk thoroughly; grease can prevent the egg whites from forming a stable meringue. Set out a hand mixer or stand mixer, a flexible spatula and a food thermometer.
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Combine the whites and sugar. Add the egg whites and caster sugar to the bowl. Whisk them together before setting the bowl over the pan so the sugar is distributed through the whites. Keep the water at a gentle simmer, not a rolling boil, and make sure steam can escape around the bowl.
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Heat the mixture safely. Whisk continuously for about 6–8 minutes, scraping the sides and bottom as you go. Check the temperature in the centre with a food thermometer; it must reach 160°F (71°C), the USDA FSIS safe minimum internal temperature for egg dishes. The sugar should also feel fully dissolved when a little mixture is rubbed carefully between two clean fingers. If it still feels gritty, continue whisking and checking the temperature, taking care not to let the whites cook against the hot bowl.
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Whip to a cool, glossy meringue. Remove the bowl from the saucepan, wipe any condensation from its underside and transfer the mixture to a stand mixer, or use a hand mixer. Whip on medium-high for about 10–12 minutes, until the meringue is thick, bright white and forms firm peaks. The bowl should feel close to room temperature before you add butter. This cooling happens as the mixer runs; if the bowl still feels warm, keep whipping rather than adding butter early.
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Add the butter gradually. Turn the mixer to medium-low and add the butter a cube at a time, allowing each addition to disappear before adding the next. The mixture may look loose or curdled partway through. Keep beating for 3–5 minutes after the last cube; it usually comes together into a smooth, fluffy frosting. Add the vanilla and salt, then beat briefly to combine.
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Check the texture. Stop the mixer and scrape down the bowl. If the frosting is smooth and holds its shape, it is ready to use. If it looks soupy, the meringue or butter may have been too warm; chill the bowl for 10 minutes and beat again. If it looks curdled and the bowl feels cool, continue beating for a few minutes. Avoid adding extra sugar to correct the texture, as it will not solve a temperature problem.
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Frost or pipe. Use the buttercream straight away for the softest spreading texture. For sharp piping, transfer it to a piping bag and work in a cool room. If it becomes too soft while decorating, pause and chill it briefly, then beat or stir it until it is pliable again.
Tips
- Keep everything clean. Egg whites whip best in a grease-free bowl. Separate each egg into a small cup before adding it to the main bowl, so a broken yolk does not spoil the whole batch. Avoid plastic bowls, which can retain a film of grease.
- Use butter at the right temperature. Butter that is fridge-cold can leave little lumps, while very soft or melted butter can make the frosting soupy. Cut it into small cubes and let it soften until it bends slightly when pressed, but still feels cool.
- Choose a stand mixer if available. A hand mixer works, but whipping the meringue takes sustained effort. Keep the beaters moving around the bowl so the whites cool evenly, and pause only briefly to check the temperature and texture.
- Adjust flavour carefully. For a less sweet impression, add a little more salt or use a flavouring such as citrus zest. Do not reduce the sugar substantially: it helps stabilise the meringue, and a large change can affect the structure as well as the taste.
Serving Suggestions
Spread the buttercream over a completely cool sponge cake, layer cake or traybake. It is particularly useful when you want a smooth finish for a celebration cake, as it can be smoothed with a palette knife and will hold piped shells, rosettes and borders. For cupcakes, fit a star nozzle to a piping bag and pipe from the outside towards the centre, keeping the bag upright for an even swirl.
Pair it with vanilla, chocolate, lemon or almond cake, or use it to sandwich two thin cake layers with a tart jam or fruit curd. A small amount of a contrasting filling adds flavour without making the buttercream itself overly sweet. Allow a refrigerated, frosted cake to lose its chill before serving if the room is cool; the buttercream tastes and spreads better when it is not firm straight from the fridge.
Variations
Chocolate: Melt 100 g (3½ oz) dark chocolate and let it cool until just fluid, then beat it into the finished buttercream. It should not be warm, or it can soften the buttercream. The result is richer and slightly firmer once cool. For a gentler chocolate flavour, use 75 g (2½ oz) milk chocolate instead.
Citrus: Beat in 1–2 tsp (5–10 ml) finely grated lemon or orange zest. Zest gives a clear citrus aroma without adding much liquid. For a sharper finish, add up to 1 tbsp (15 ml) strained juice a little at a time; too much juice can loosen the frosting.
Coffee: Dissolve 1 tbsp (6 g) instant espresso powder in 1 tsp (5 ml) hot water, cool it completely, then beat it in gradually. This gives a coffee buttercream that works well with chocolate or plain sponge. Add a little at a time and stop if the frosting begins to lose its shape.
Vanilla bean: Replace the extract with the seeds scraped from one vanilla pod, or use 1 tsp (5 ml) vanilla bean paste. The tiny seeds give the frosting a speckled appearance. For other extracts, start with ½ tsp (2.5 ml), taste, and add more only if needed because some flavours are more concentrated than vanilla.
Storage and Reheating
Because this frosting contains egg and butter, refrigerate it within 2 hours of making it or serving it. Store it in an airtight container in the fridge for up to 4 days. If it is already on a cake, cover the cake well and refrigerate; delicate decorations may flatten if they touch the cover. Avoid leaving the buttercream at room temperature for more than 2 hours, especially in a warm room.
For longer storage, freeze the buttercream in an airtight freezer-safe container for up to 2 months for best quality. Thaw it overnight in the refrigerator. Do not microwave or otherwise reheat it: warmth can melt the butter and split the emulsion. Instead, let chilled buttercream sit at room temperature until pliable, then beat it on medium speed until smooth. If it looks curdled after thawing, keep mixing; if it is too soft, chill it for 10 minutes and beat again.
Frosted cakes can also be refrigerated or frozen, though the finish may be less pristine after thawing. For the best texture, bring the cake out of the fridge with its cover on and let it soften gradually, then remove the cover before serving. Do not refreeze buttercream that has been thawed and left out for an extended period.
Frequently Asked Questions
Why has my Swiss meringue buttercream turned runny?
The meringue may still have been warm when the butter went in, or the butter may have been too soft. Chill the bowl for about 10 minutes, then beat again. If the mixture is runny but cool, a short chill followed by steady mixing can help it emulsify. Do not add more butter while it is warm.
Why does it look curdled?
A curdled appearance is usually a temporary sign that the meringue and butter are at different temperatures. If the bowl feels cool, keep beating for several minutes; the mixture often becomes smooth as it emulsifies. If it feels warm and loose, chill it briefly first, then resume beating.
Can I make Swiss meringue buttercream with salted butter?
Yes. Use the same quantity of salted butter and leave out the added salt at first. Taste the finished frosting and add a pinch only if you want a more pronounced salty contrast. Salt levels vary between butter brands, so adding it gradually gives you better control.
Can I use carton egg whites?
Check the carton instructions before using them. Some pasteurised carton whites do not whip as reliably as fresh shell whites, and products can differ in composition. If the label says they are suitable for whipping, they may work, but expect the meringue to take longer or have a slightly different volume. Still heat the mixture to 160°F (71°C) and verify it with a thermometer.
Is Swiss meringue buttercream suitable for decorating ahead?
Yes. Make it up to 4 days ahead and refrigerate it airtight, or freeze it for up to 2 months. Before piping or spreading, bring it back to a pliable temperature and beat it until smooth. For a large cake, you can frost it ahead and keep it covered in the refrigerator, then allow it to soften gradually before serving.
How can I tell whether the sugar has dissolved?
Rub a small amount of the heated mixture between clean fingers after removing the bowl from the heat; it should feel smooth rather than sandy. Take care because the mixture is hot. If you feel crystals, return the bowl to the pan and whisk a little longer, checking the temperature again before whipping.
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Safety notes: safe minimum internal temperatures from USDA FSIS; raw and undercooked fish guidance from the U.S. FDA. Food safety notes
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