Graduation Sheet Cake With Vanilla Buttercream

This graduation cake is a white vanilla sheet cake baked in a 9×13-inch pan and spread with vanilla buttercream, so the whole top becomes a flat canvas for a fondant cap, a diploma, school colors or the graduate’s name. It serves 16, takes about 20 minutes to mix and 25 minutes to bake, and uses egg whites, cake flour and sour cream for a soft, pale crumb that still holds up under decorations. Below you will find what each ingredient does, the method, how to plan the decorating around party day, and what to change if something goes wrong.

Ingredients

For the Cake

• 2 1/2 cups cake flour
• 2 teaspoons baking powder
• 1/2 teaspoon baking soda
• 1 teaspoon salt
• 3/4 cup unsalted butter, softened
• 1 3/4 cups granulated sugar
• 4 large egg whites
• 1/2 cup sour cream
• 1 tablespoon vanilla extract
• 1 cup whole milk

For the Buttercream

• 1 cup unsalted butter, softened
• 4 cups powdered sugar
• 2-3 tablespoons milk
• 2 teaspoons vanilla extract
• Pinch of salt

Optional Graduation Decorations

• Black fondant for a graduation cap
• Candy tassels or licorice strings
• White fondant rolled into a diploma shape
• Sprinkles in school colors
• Lettering gel for writing the graduate’s name or class year

What the Key Ingredients Do

• Cake flour has less protein than all-purpose flour, which keeps the crumb fine and tender.
• Egg whites only keep the cake white, so school-color frosting and sprinkles stand out against the slice.
• Sour cream adds moisture and a little tang, and helps the cake stay soft if you bake it a day ahead.
• Softened butter should dent when pressed but not look greasy. Butter that is too warm makes a heavy cake and a runny buttercream.

How to Bake and Frost the Sheet Cake

1. Preheat the oven to 350°F. Grease and line a 9×13-inch baking pan. Whisk the flour, baking powder, baking soda and salt together until evenly combined.

2. In a large bowl, beat the butter and sugar for 3 to 4 minutes until the mixture looks pale and fluffy. This traps air and gives the cake much of its lift.

3. Mix in the egg whites one at a time, followed by the sour cream and vanilla. Scrape down the bowl so no butter streaks remain.

4. Add the dry ingredients and milk alternately, beginning and ending with the dry mixture. Stir only until no streaks of flour remain; overmixing makes the cake tough and domed.

5. Spread the batter evenly into the prepared pan, pushing it slightly into the corners. Bake for about 25 minutes, until the top springs back lightly when touched, a toothpick from the center comes out with a few moist crumbs, and the edges start to pull away from the pan.

6. Cool the cake completely before frosting. A warm cake melts the buttercream and makes decorations slide.

7. For the frosting, beat the butter until smooth. Add the powdered sugar gradually, then mix in the vanilla, salt and enough milk to make it spreadable. It should hold soft peaks.

8. Frost the top of the cooled cake in an even layer. Decorate with a fondant graduation cap, a diploma, school colors or a simple "Class of" message.

9. Chill the cake for 20 minutes before slicing if you want especially clean edges.

Decorating a Graduation Cake Without Stress

• Crumb coat first. Spread a very thin layer of buttercream, chill it for 20 minutes, then add the final layer. Loose crumbs stay trapped under the first coat.
• Sketch the lettering. Mark the name or year lightly with a toothpick before piping, so spacing mistakes are easy to smooth over.
• Keep toppers in proportion. A cap or diploma that covers only a small corner of the cake reads clearly and leaves most slices easy to cut.
• Tint only what you need. Color a small portion of the buttercream for borders and writing. Deep shades such as navy or red need a lot of gel coloring and can taste bitter if used over the whole cake.
• Keep it simple. A clean white top with a border in school colors and one topper usually looks neater than many small decorations.

Why the Cake Turns Out Dry or Dense

• Dry: Overbaking is the most common cause. Start checking a few minutes before the 25-minute mark, and take the cake out as soon as the center springs back.
• Dense or gummy: Usually cold ingredients or overmixing after the flour goes in. Let the egg whites, sour cream and milk come to room temperature, and stop mixing as soon as the flour disappears.
• Sunken center: Opening the oven too early or underbaking. Wait until the edges have set before checking.
• Grainy buttercream: Unsifted powdered sugar or too little beating. Sift the sugar and keep beating for a couple of minutes after the last addition.

Swaps and What to Expect

• All-purpose flour instead of cake flour: The cake will be slightly denser and less fine-crumbed, but still fine for a sheet cake.
• Plain Greek yogurt instead of sour cream: Expect a similar moist crumb with a bit more tang.
• Different flavor: Replacing part of the vanilla in the buttercream with a little almond extract or lemon zest changes the flavor without changing the texture.
• Store-bought frosting: Expect a sweeter, softer frosting that is harder to pipe crisp lettering with; chilling it for a few minutes helps.

Planning Ahead for the Party

Bake the cake one day ahead, cool it completely, and keep it tightly covered at room temperature. Frost and decorate it on the day of the party, or the evening before if you keep it in the refrigerator. Because the buttercream is mostly butter, keep the decorated cake cool and out of direct sun at an outdoor party, and let a refrigerated cake stand at room temperature for about 30 minutes before serving so the frosting softens.

The unfrosted cake freezes well for up to 2 months. Wrap it tightly in plastic wrap and thaw it overnight, still wrapped, before frosting. Leftover frosted cake keeps covered for a few days; refrigerate it if the room is warm.

Rounding Out the Dessert Table

A sheet cake is the centerpiece, but smaller treats help when guests arrive at different times. Graduation cupcakes and decorated graduation cookies can carry the same school colors as the cake. For the savory side of an open-house party, baked ham and cheese sliders are easy to make in batches, and a pitcher of strawberry basil lemonade works for guests of all ages.

FAQ

How many people does this graduation cake serve?

The 9×13-inch sheet cake cuts into 16 generous party slices. For a larger crowd, cut smaller squares or add cupcakes and cookies to the table.

How do I keep the decorations from sliding?

Chill the frosted cake for about 20 minutes before adding fondant decorations. A firmer frosting surface keeps toppers in place.

Can I write the name in buttercream instead of gel?

Yes. Tint a few spoonfuls of the buttercream, spoon it into a piping bag with a small round tip, and pipe over your toothpick sketch. Chilled buttercream gives cleaner lines.

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