Preheat oven to 350°F/175°C. Butter and flour an 18x13x1.5-inch sheet pan, or spray with a nonstick baking spray.
In a large bowl, cream together the butter and sugar with an electric or stand mixer on high speed, until light and fluffy. On medium speed, add the eggs, two at a time until incorporated. Then mix in the sour cream and vanilla. Scrape down the sides of the bowl and stir until smooth.
In a medium bowl, whisk together the flour, cornstarch, salt, and baking soda. Add the flour mixture to the butter mixture and stir until just combined. Do not over mix. Pour the batter into the prepared pan. Smooth the top with a spatula.
Bake for 25-35 minutes, until a toothpick comes out clean and cake is golden brown. Let the cake cool to room temperature.
For the frosting, combine the butter, cream cheese, sugar, and vanilla in a large bowl with an electric mixer until smooth. Spread about 2/3 of the frosting on top of the cooled cake. Use a toothpick to outline the flag on the frosted cake.
Fill the upper left corner with blueberries. Use sliced or diced strawberries for the red stripes. Fill a pastry bag with a star tip with the remaining frosting and pipe the white stripes between the rows of strawberries. Pipe the stars between the blueberries.
Cut into squares and serve. Refrigerate leftovers.
Reduce cake ingredients by half to make it in a 9x13 pan.