01 - Preheat oven to 350°F. Line a 9x13-inch baking pan or large baking sheet with parchment paper.
02 - In a large bowl, cream the softened butter and granulated sugar until pale and fluffy. Add egg and vanilla extract, beating thoroughly to incorporate.
03 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt.
04 - Gradually add dry ingredients to the wet mixture, blending just until a cohesive dough forms.
05 - Press dough evenly into the prepared pan to a thickness of approximately 1/4 inch. Bake for 13 to 15 minutes, or until the edges are lightly golden. Remove and allow to cool completely in the pan.
06 - With an electric mixer, beat softened cream cheese and butter until smooth. Incorporate powdered sugar and vanilla extract, mixing until the frosting is creamy.
07 - Spread the cream cheese frosting in an even layer over the fully cooled sugar cookie crust.
08 - Arrange blueberries in the top left section to resemble stars on a flag. Create stripes with sliced strawberries and raspberries, alternating with the white frosting to complete the flag design.
09 - Refrigerate for at least 30 minutes to set. Once chilled, slice and serve.