Preheat the oven to 350°F. Butter and flour an 18x13 by 1½-inch sheet pan.
MAKE THE CAKE:
In a large bowl, use an electric mixer to cream the butter and sugar on high speed, until light and fluffy. On medium speed, add the eggs, 2 at a time, then add the sour cream and vanilla. Scrape down the sides and stir until smooth.
In a medium bowl, sift together the flour, cornstarch, salt, and baking soda. Blend the flour mixture into the butter mixture until just combined. Pour into the prepared pan. Smooth the top with a spatula. Bake in the center of the oven for 20 to 30 minutes, until a toothpick comes out clean. Cool to room temperature.
ADD THE FROSTING AND FLAG DECORATION:
In a large bowl, use an electric mixer to combine the butter, cream cheese, sugar, and vanilla, mixing just until smooth.
Spread three-fourths of the frosting on the top of the cooled sheet cake. Outline the flag on the top of the cake with a toothpick. Fill the upper left corner with blueberries. Place 2 rows of strawberries across the top of the cake like a red stripe. Put the remaining frosting in a pastry bag fitted with a star tip and pipe two rows of white stripes below the strawberries. Alternate rows of strawberries and frosting until the flag is completed. Pipe stars on top of the blueberries.