01 - Preheat oven to 350°F. Line a 9x13 inch baking pan or large baking sheet with parchment paper.
02 - In a large mixing bowl, blend softened butter with granulated sugar using an electric mixer until light and fluffy. Beat in egg and vanilla extract until well incorporated.
03 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt. Gradually add dry mixture to wet ingredients, mixing until a soft dough forms.
04 - Press dough evenly into prepared pan to a 1/4 inch thickness. Bake for 13-15 minutes until edges are golden. Cool completely on a rack.
05 - In a clean bowl, beat softened cream cheese and butter until smooth. Add powdered sugar and vanilla extract, mixing until frosting is creamy.
06 - Spread cream cheese frosting evenly over the cooled sugar cookie crust using a spatula.
07 - Layer blueberries in the upper left corner to resemble the flag's stars, then alternate strawberries and raspberries to form red stripes, leaving spaces of frosting for white stripes.
08 - Refrigerate assembled dessert for at least 30 minutes before slicing and serving.