Instructions

1. 1

   Preheat the oven to 350°F. Grease and line a 9x13-inch baking pan. Whisk the flour, baking powder, baking soda, and salt together until evenly combined.

2. 2

   In a large bowl, beat the butter and sugar for 3 to 4 minutes until the mixture looks pale and fluffy. This step helps create a soft crumb in the finished graduation sheet cake.

3. 3

   Mix in the egg whites one at a time, followed by the sour cream and vanilla. The batter may look slightly silky and glossy at this stage.

4. 4

   Alternate adding the dry ingredients and milk, beginning and ending with the dry mixture. Stir only until no streaks of flour remain.

5. 5

   Spread the batter evenly into the prepared pan. Bake for 24 to 28 minutes, or until the top springs back lightly when touched and the edges begin pulling away from the pan.

6. 6

   Cool completely before frosting. A warm cake can melt the buttercream and make decorating difficult.

7. 7

   For the frosting, beat the butter until smooth. Add powdered sugar gradually, then mix in the vanilla, salt, and enough milk to create a spreadable texture. The frosting should hold soft peaks.

8. 8

   Frost the cooled cake in an even layer. Decorate with a fondant graduation cap, diploma cake design, school colors, or a simple "Class of 2026" message.

9. 9

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