Heat oven to the temperature listed on the cake mix box and bake the 2 cake mixes in a large 12x18 sheet pan or in two 9x13 pans joined together, then cool completely. This gives a firm sheet that holds a smooth frosting layer without tearing.

Let the cakes cool fully before frosting, about 1 hour at room temperature. Cooling completely prevents the buttercream from melting and keeps the fruit flag lines sharp.

Beat the softened unsalted butter until fluffy, then gradually add powdered sugar, vanilla extract, and 4–6 tbsp heavy cream. Mix until smooth and spreadable so it can fill gaps between fruit slices.

Spread a thick, even layer of white buttercream over the entire top surface of the cooled sheet cake. Smooth the top so the flag design looks flat and vivid from above.

In the upper left corner, arrange the fresh blueberries into a dense rectangle to form the blueberry canton. Press gently so the pieces touch and the rectangle looks precise.

Create the red stripes by arranging strawberry slices in rows across the length of the cake. Keep the rows uniform so the stripes read clearly in the finished design.

Fill the white stripes by piping extra frosting in rows between the strawberry rows or by placing thin banana slices. Choose one method for clean contrast and neater separation between red and white bands.

Refrigerate the cake until ready to serve. Chilling helps the buttercream set and keeps fruit from sliding when you slice.

Slice into squares and serve cold. Use a sharp cut to maintain straight stripes and a clean blueberry canton.