Key lime frosting is a creamy butter-and-cream-cheese icing flavored with fresh lime zest and juice, ideal for cupcakes, cakes, and cookies. Its sweet, tangy balance brings a fresh citrus note to rich bakes, and a little juice goes a long way without making the frosting runny.
Ingredients
- 1 cup (227 g) unsalted butter, softened but still cool
- 1 package (8 oz / 226 g) full-fat block cream cheese, softened but still cool
- 4 cups (480 g) powdered sugar, sifted if lumpy
- 1 tablespoon (about 6 g) finely grated key lime zest, from about 3 to 4 key limes
- 1½ tablespoons (22 ml) key lime juice, freshly squeezed and strained
- 1 teaspoon (5 ml) vanilla extract
- ⅛ teaspoon (0.7 g) fine salt
- 1 to 2 teaspoons (5 to 10 ml) heavy cream or milk, only if needed to adjust the consistency
Instructions
- Prepare the citrus and dairy. Zest the limes before cutting them, taking only the thin green outer peel and avoiding the bitter white pith. Squeeze and strain the juice to remove seeds and pulp. Let the butter and cream cheese soften until a finger presses into them easily, but use them while they still feel cool rather than oily or warm. This helps the frosting stay thick enough to spread or pipe.
- Beat the butter. Put the butter in a large mixing bowl and beat with a hand mixer or stand mixer fitted with the paddle attachment on medium speed for about 2 minutes, until smooth and creamy. Scrape down the bowl and beater once. Starting with butter on its own helps avoid lumps in the finished frosting.
- Add the cream cheese. Add the softened cream cheese in several pieces and beat on medium-low just until the mixture is smooth, about 30 to 60 seconds. Scrape the bowl as needed. Stop as soon as no streaks remain; extended beating can warm the mixture and make cream cheese frosting softer than you want.
- Mix in the sugar. Add the powdered sugar in three or four additions, mixing on low speed after each one so it does not puff out of the bowl. Once incorporated, add the zest, 1 tablespoon (15 ml) of the lime juice, vanilla, and salt. Beat on low until combined, then on medium for about 30 seconds to make the frosting fluffy. Avoid beating for several minutes, which can loosen it.
- Adjust and use. Taste the frosting. Add the remaining ½ tablespoon (7 ml) lime juice a little at a time if you want more tartness, mixing briefly after each addition. For a softer consistency, beat in 1 teaspoon (5 ml) cream or milk; add the second teaspoon only if needed. If it is too soft to pipe, refrigerate it for 15 to 20 minutes, then stir or beat briefly and frost cooled cupcakes or cake.
Tips
- Choose the right cream cheese. Full-fat block cream cheese gives the frosting a sturdier texture. Tub-style spreadable cream cheese contains more moisture and can make the icing slack, especially if you want defined piping. Let the block soften enough to blend, but do not leave it somewhere warm until it turns glossy or runny.
- Add lime juice gradually. The zest supplies a vivid lime aroma, while juice provides the tart flavor and extra liquid. Start with the amount in the recipe, taste, and increase in small increments. Adding a large splash at once may make the frosting too loose to hold a swirl.
- Use zest for stronger citrus flavor. If you want more lime fragrance without thinning the frosting, add another teaspoon of finely grated zest rather than a lot more juice. A fine grater makes small pieces that distribute evenly and feel pleasant in the icing.
- Keep the mixer speed moderate. Use low speed when adding sugar and stop beating once the frosting is smooth. If it becomes soft, chill it briefly rather than trying to fix it with a large amount of extra powdered sugar, which can make it overly sweet and heavy.
Serving Suggestions
This batch makes about 3 cups of frosting, enough for 12 cupcakes with generous swirls or a thinner layer on a small cake. Spread it over completely cooled vanilla, coconut, or chocolate cupcakes; frosting warm cake melts the butter and can cause the icing to slide. For a layer cake, use it between layers and on top, or pair it with a less-sweet vanilla buttercream on the sides if you prefer a lighter coating.
Key lime frosting also works on sugar cookies, shortbread bars, and simple loaf cake. For a clean finish, spread it with an offset spatula and scatter a little extra finely grated zest over the top just before serving. Toasted coconut, thin lime slices, or a few crushed graham crackers make fitting garnishes, but add wet or crunchy toppings close to serving so they do not soften the frosting.
Because the frosting contains cream cheese, plan to keep frosted desserts chilled when they are not being served. Let a cake sit at room temperature for about 20 to 30 minutes before serving so the frosting softens slightly and the lime flavor is easier to taste. Avoid leaving it out for a long gathering; return leftovers to the refrigerator within 2 hours.
Variations
Use regular limes. Key limes are small, aromatic citrus fruits with a bright, tart flavor, but regular Persian limes are a practical substitute. Use the same measured amount of juice and zest. The result will still be lime frosting, though it may taste a little less floral and more familiar or sharp. Bottled key lime juice can also work when fresh key limes are unavailable; use the same amount, taste before adding more, and keep the fresh zest if possible for better aroma.
Make it more lime-forward. Add up to 2 teaspoons of extra zest for a more pronounced citrus scent. Keep extra juice modest and add it by the teaspoon, since a thinner frosting may not hold piping details. For a subtle color, a tiny amount of green food coloring can be mixed in, but it is entirely optional; the natural frosting is usually pale cream rather than bright green.
Make a simpler buttercream. For a frosting without cream cheese, leave it out and use 1½ cups (340 g) unsalted butter instead of the 1 cup listed. Keep the powdered sugar, zest, vanilla, and salt, and begin with 1 tablespoon (15 ml) lime juice. Add a little cream or milk as needed to make it spreadable. This version is sweeter and more buttery, with a less tangy, less creamy finish.
Add a graham-cracker note. For a dessert reminiscent of key lime pie, sprinkle crushed graham crackers over the frosting just before serving, or press them onto the sides of a frosted cake. Mixing a large quantity into the frosting can make its texture gritty and may soften the crumbs, so use them as a topping instead.
Storage and Reheating
Transfer unused frosting to an airtight container and refrigerate it for up to 4 days. Refrigerate frosted cupcakes or cake within 2 hours, and keep them covered so the frosting does not absorb odors or dry out. The frosting will firm in the refrigerator; let it stand at room temperature for about 20 to 30 minutes, then stir or beat it briefly until spreadable. If it seems too soft after coming to temperature, chill it again for 10 to 15 minutes.
You can freeze the frosting in an airtight freezer-safe container for up to 1 month. Thaw it overnight in the refrigerator, then let it soften briefly at room temperature and beat on low just until smooth. Do not microwave or otherwise heat it: warmth can melt the butter and separate the cream cheese, leaving the frosting greasy or runny. Once thawed, use it promptly and do not refreeze it.
Frequently Asked Questions
Can I make key lime frosting without fresh key limes?
Yes. Regular limes work well in the same measured quantities. Bottled key lime juice is another option, but it does not provide the fresh aroma of zest, so add finely grated zest from regular limes if you can. Strain fresh juice to keep seeds and pulp out of the frosting.
Why is my frosting too runny?
The butter or cream cheese may have been too warm, the mixture may have been beaten for too long, or too much juice may have been added. Chill the bowl for 15 to 20 minutes and mix briefly. If it still needs structure, add powdered sugar a few tablespoons at a time, tasting as you go; a small amount of additional sugar is better than adding several cups at once.
Can I pipe this frosting?
Yes, if it is cool and firm enough. Use block cream cheese, measure the lime juice carefully, and avoid adding optional cream unless the frosting is genuinely too stiff. Chill it briefly if needed, then pipe soon after it becomes workable. It is a soft cream-cheese frosting, so it will not hold intricate, sharply defined decorations as firmly as a stiff buttercream.
Can I make it ahead?
Yes. Make it up to 4 days ahead and refrigerate it in a covered container. Before using, let it soften for about 20 to 30 minutes and beat it briefly until smooth. You can also freeze it for up to 1 month; thaw it in the refrigerator overnight before bringing it to a spreadable consistency.
Is this frosting very tart?
It is sweet with a clear but moderate lime tang. The cream cheese and powdered sugar soften the sharpness of the juice, while zest adds citrus aroma. For a milder flavor, begin with 1 tablespoon of juice; for a brighter finish, add the remaining measured juice gradually and taste before deciding whether you need more.
How much frosting does this recipe make?
It yields about 3 cups, depending on how much air is beaten into it and whether you add the optional cream. That is enough for 12 cupcakes with substantial swirls, more cupcakes with a thinner layer, or a modest coating on a small cake. For a large or heavily decorated layer cake, make one and a half or two batches.
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