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.
For the icing, combine the butter, cream cheese, and vanilla in the bowl of an electric mixer fitted with the paddle attachment and mix on medium speed until very smooth. Add the confectioners’ sugar and mix on low speed. Stir with a rubber spatula to be sure the icing is well mixed.
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 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 raspberries and 2 rows of icing until the flag is completed. Pipe stars on top of the blueberries.
Note: If you don’t want to serve your cake from the pan, line the pan with parchment paper and turn it out into a board before you ice and decorate it.
Copyright 2002, Barefoot Contessa Family Style, Clarkson Potter/Publishers, All Rights Reserved