Heat the oven to 350 degrees. Butter and flour an 18 x 12 x 1-1/2-inch sheet pan (see note).

Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment for 3 minutes on medium speed, until light and fluffy. (If the butter is too cool to be light and fluffy, add a teaspoon or two of warm water,) On medium-low speed, add the eggs, 2 at a time, then add the sour cream and vanilla. Scrape down the sides and stir until smooth.

Sift together the flour, cornstarch, salt, and baking soda in a bowl. With the mixer on low, add the flour mixture and mix until just combined. Stir the batter with a rubber spatula and pour into the prepared pan. Smooth the top and bake in the center of the oven for 20 to 30 minutes, until a toothpick comes out clean. Cool to room temperature.

Frosting:

• Combine butter and cream cheese in the bowl of a stand mixer or use an electric mixer. Beat until creamy, well-combined, and smooth.
• 2. Add vanilla extract and salt and stir well to combine.
• With mixer on low, gradually add powdered sugar until completely combined.
• Use to frost completely cooled cake.
• Flag Design:
• Spread three quarters of the icing on the cake. Using a toothpick, outline a flag on the cake. Fill in the upper left corner with blueberries. Place 2 rows of strawberries or raspberries across the top of the cake like the red stripes. Put the remaining icing in a pastry bag fitted with a star tip and pipe two rows of white stripes below the raspberries. Alternately add 2 rows of strawberries or raspberries and 2 rows of icing until the flag is completed. Pipe stars on top of the blueberries.