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American Flag Cake

American flag cake made as a patriotic sheet cake with a thick white buttercream base and a crisp, clean blueberry canton with strawberry red stripes. This red white blue cake is assembled on a full sheet for easy slicing into squares.
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Prep Time 45 minutes
Cook Time 35 minutes
cooling 1 hour
Total Time 2 hours 20 minutes
Servings: 20 servings
Course: Dessert
Cuisine: American
Ingredients Equipment Method Notes

Ingredients
  

White cake
  • 2 boxes white cake mix Use the box directions for eggs, oil, and water (include the specific amounts from your box in box instructions).
Buttercream frosting
  • 2 cups unsalted butter Softened.
  • 6 cups powdered sugar
  • 2 tsp vanilla extract
  • 4 tbsp heavy cream Add 4–6 tbsp as needed for spreadable consistency.
Blueberry canton
  • 2 cups fresh blueberries
Strawberry red stripes
  • 2 lb fresh strawberries Hulled and sliced lengthwise.
White stripe option
  • 1 banana slices Use banana slices for the white stripes, or substitute extra white frosting.

Equipment

  • 1 sheet pan

Method
 

Bake the sheet cake
  1. Bake both white cake mixes in a large 12x18 sheet pan or two 9x13 pans joined together according to package directions, then cool completely.
  2. Let the cakes cool fully before frosting so the buttercream spreads smoothly without melting.
Make the white buttercream
  1. Beat the softened unsalted butter until fluffy.
  2. Gradually add powdered sugar and beat until the mixture is combined and looks pale.
  3. Mix in the vanilla extract and then add 4 tbsp heavy cream.
  4. Beat until smooth and spreadable, adding 1–2 more tbsp heavy cream if needed to reach a thick, pipeable texture.
Frost and decorate the flag
  1. Spread a thick, even layer of buttercream over the entire cooled sheet cake top.
  2. In the upper left corner, arrange fresh blueberries into a dense rectangle to form the canton.
  3. Create red stripes by arranging sliced fresh strawberries in flat rows across the length of the cake.
  4. Fill the white stripes by piping extra frosting in rows between the strawberry rows, or placing thin banana slices to form clean white lines.
Chill and serve
  1. Refrigerate the cake until ready to serve, then slice into squares.

Notes

Pro tip: For the cleanest flag design, keep strawberry rows uniform in thickness and place them firmly so the buttercream stays smooth under the fruit. Refrigerate in an airtight container for up to 3 days; freeze the unfrosted cake layers for up to 2 months (freeze baked cake only, then thaw and frost). Dietary swap: For a lighter dessert, use a reduced-fat frosting alternative or replace part of the butter with a baking butter substitute, but expect a softer texture.