01 - Preheat oven to 350°F. Line a large baking sheet with parchment paper or coat a 9x13-inch baking pan with non-stick spray.
02 - In a large mixing bowl, beat softened butter with granulated sugar until light and fluffy. Add egg and vanilla extract, mixing until smooth.
03 - In a separate bowl, whisk together flour, baking powder, and salt. Gradually add dry ingredients to wet mixture until just combined.
04 - Press dough evenly into the prepared pan to achieve a crust about 1/4 inch thick. Bake for 13 to 15 minutes until edges are lightly golden. Cool completely in the pan.
05 - Using an electric mixer, blend the softened cream cheese and butter until smooth. Add powdered sugar and vanilla, mixing until creamy and homogenous.
06 - Spread the cream cheese frosting evenly over the cooled cookie crust with a spatula.
07 - Arrange blueberries in the top left corner to mimic the stars section of a flag. Layer sliced strawberries and raspberries in alternating rows to form the red stripes, leaving frosting exposed for the white stripes.
08 - Refrigerate assembled dessert for at least 30 minutes. Slice and serve chilled.