This 10 x 15 sheet cake is a tender vanilla butter cake baked in a shallow pan and finished with a simple chocolate frosting. It suits birthdays, potlucks, and casual gatherings: the batter is easy to mix, the cake bakes in about half an hour, and the broad pan makes neat portions without stacking layers.
Ingredients
- 2 1/2 cups (300 g) all-purpose flour, spooned into the measuring cups and leveled
- 1 3/4 cups (350 g) granulated sugar
- 2 teaspoons baking powder
- 1/2 teaspoon baking soda
- 3/4 teaspoon fine salt
- 3/4 cup (170 g) unsalted butter, softened
- 3 large eggs (about 150 g without shells), at room temperature
- 1 tablespoon (15 ml) vanilla extract
- 1 cup (240 ml) buttermilk, at room temperature
- For the frosting: 1/2 cup (113 g) unsalted butter
- 1/3 cup (30 g) unsweetened cocoa powder
- 3 cups (360 g) powdered sugar, sifted if lumpy
- 1/3 cup (80 ml) milk, plus 1–2 tablespoons (15–30 ml) as needed
- 1 teaspoon (5 ml) vanilla extract
- 1/8 teaspoon fine salt
Instructions
- Set a rack in the middle of the oven and heat it to 350°F (175°C). Grease a 10 x 15-inch rimmed baking pan, then line the bottom with parchment paper if you want the cake to release easily. The pan should be at least 1 inch deep; do not use a flat baking sheet.
- In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. In a large bowl, beat the softened butter and sugar with a hand mixer or stand mixer on medium speed for 2–3 minutes, until pale and fluffy. Scrape down the bowl.
- Beat in the eggs one at a time, mixing until each is incorporated. Mix in the vanilla. If the mixture looks slightly curdled, it should come together when the dry ingredients and buttermilk are added.
- Add about one-third of the flour mixture and mix on low just until no dry flour remains. Add half the buttermilk, then another third of the flour, the remaining buttermilk, and the remaining flour. Mix each addition briefly; stop as soon as the batter is smooth. Scrape the bowl and fold once or twice with a spatula, without beating the batter further.
- Scrape the batter into the prepared pan and spread it into an even layer, reaching the corners. Bake for 25–30 minutes, rotating the pan once after about 18 minutes if your oven browns unevenly. The cake is ready when the top springs back lightly and a toothpick inserted near the center comes out clean or with a few moist crumbs. If checking with a thermometer, the center should reach about 200–205°F (93–96°C); this is a doneness cue, not a meat-safety temperature.
- Set the pan on a wire rack and let the cake cool completely, about 35–45 minutes. For the frosting, melt the butter in a small saucepan over low heat or in a microwave-safe bowl in short bursts. Whisk in the cocoa until smooth, then remove from the heat.
- Whisk the powdered sugar, 1/3 cup milk, vanilla, and salt into the cocoa mixture. Add a little more milk if necessary for a thick but spreadable frosting. Spread it over the completely cooled cake. Let the frosting sit for 10–15 minutes before cutting for cleaner slices.
Tips
- Measure flour by spooning it into the cup and leveling the top, or weigh 300 g. Scooping directly from the bag can pack in extra flour and make the cake dry.
- Room-temperature butter, eggs, and buttermilk blend more evenly. If you need to warm eggs quickly, place them in a bowl of warm—not hot—water for 5–10 minutes.
- No buttermilk? Measure 1 tablespoon (15 ml) lemon juice or white vinegar into a measuring cup, add milk to reach 1 cup (240 ml), and leave it for 5 minutes. This substitute works here, though the flavor may be a little less tangy.
- Do not frost a warm cake: the butter in the icing will melt and slide. Keep mixing gentle once the flour goes in; overmixing can make the crumb firm.
Serving Suggestions
Cut the cake into 20 squares for a gathering, or make smaller pieces when serving alongside other desserts. Serve at room temperature with coffee, tea, or cold milk. Fresh berries or sliced strawberries make a bright accompaniment, while a small scoop of vanilla ice cream turns a square into a plated dessert. For tidy portions, use a long knife and wipe the blade between cuts.
Variations
For vanilla frosting, omit the cocoa and increase the powdered sugar to 3 1/2 cups (420 g); add milk gradually until spreadable. For a sprinkle cake, fold 1/2 cup (about 80 g) of rainbow sprinkles into the finished batter. To add citrus, mix 1 tablespoon finely grated lemon or orange zest into the sugar before creaming it with the butter. You can also scatter 1 cup (170 g) chocolate chips over the batter before baking; press them lightly into the surface so they stay in place.
Storage and Reheating
Because the cake has a butter-based frosting, cover it and refrigerate leftovers within 2 hours of serving. It keeps well in the refrigerator for 3–4 days; let slices stand at room temperature for 20–30 minutes before serving for a softer crumb. For longer storage, freeze individual unfrosted or frosted portions in an airtight container for up to 2 months, separating layers with parchment. Thaw overnight in the refrigerator or for a few hours at room temperature. Cake is usually best served at room temperature; if you prefer it warm, microwave one slice for about 8–12 seconds, checking that it does not overheat.
Frequently Asked Questions
Is a 10 x 15 pan the same as a jelly-roll pan?
A common jelly-roll pan measures about 10 x 15 inches, but pan dimensions and depths vary. Check the inside measurements and use a rimmed pan at least 1 inch deep. This recipe is written for that size, not a larger 12 x 17-inch half-sheet pan.
Can I use a glass pan?
Yes, provided it measures 10 x 15 inches and has a rim. Glass may heat differently from metal, so begin checking a few minutes early and rely on the cake’s doneness cues. Avoid moving a hot glass pan onto a cold or wet surface.
Can I make the cake a day ahead?
Yes. Bake and cool it fully, then cover it tightly and refrigerate, frosted or unfrosted. Allow it to come toward room temperature before serving. The frosting can also be made ahead and gently warmed until spreadable, stirring well before use.
Can I double this recipe?
For a larger gathering, make two separate batches and bake them in two 10 x 15-inch pans. Doubling the batter into one pan would make the layer too deep and would require a different baking time and pan size.
Why did my cake turn out dry or dense?
Too much flour, overmixing, or baking beyond the point of doneness are common causes. Weigh the flour when possible, mix only until combined after adding it, and begin checking at 25 minutes because ovens and pans vary.
Can I make it without a mixer?
Yes. Use a sturdy whisk and a flexible spatula, and make sure the butter is soft enough to cream smoothly with the sugar. The texture may be slightly less airy, but careful mixing and room-temperature ingredients still produce a good cake.
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