How to Make It
Spray a jelly roll pan (10- x 15-inch) with nonstick cooking spray. Preheat oven to 350˚. In a large bowl, whisk together the flour, baking powder, salt, and sugar.
In a separate bowl, using an electric mixer on medium-low speed, beat together the eggs, oil, sour cream, and vanilla. On low speed, beat in half the dry ingredients, just until combined. Beat in the remaining dry ingredients, taking care not to overbeat the batter.
Scrape the batter into the prepared pan and spread evenly with a spatula. Bake for 20 to 25 minutes, or until the cake is golden and a skewer inserted into the center of the cake comes out clean. Transfer cake to a cooling rack and cool completely.
FOR FROSTING: In a stand mixer, or with a hand-held electric mixer, beat the butter and half the powdered sugar together until light and fluffy. Beat in the sour cream until smooth, and then beat in the remaining powdered sugar until the frosting is thick and spreadable. Fold in the vanilla.
Spread half the frosting over the cooled cake, making sure it is very smooth. Fill a piping bag fitted with a large star tip with the remaining frosting.
Fill the upper left hand corner with a 5-inch square of blueberries. Try to pick strawberries that are relatively small and the same size. Remove the stem and slice them in half. Starting from the top of the cake, place two rows of strawberries (one row with the points pointing the right, and underneath a second row with the points pointing to the left). Pipe a shell border of frosting underneath the berries. Continue, alternating the double row of berries with a single shell border of frosting until the flag is complete.
Refrigerate until ready to serve. Cut and serve straight from the pan.