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, beat softened butter with granulated sugar on medium speed until light and fluffy. Add egg and vanilla extract, blending until smooth.
03 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt. Gradually incorporate dry mixture into the butter mixture until a soft dough forms.
04 - Press dough evenly into the prepared pan to a thickness of about 1/4 inch, ensuring even coverage.
05 - Bake for 13 to 15 minutes until edges are lightly golden. Remove and allow to cool completely in the pan.
06 - Using an electric mixer, cream softened cream cheese and butter until smooth. Add powdered sugar and vanilla extract; beat until light and creamy.
07 - Spread the cream cheese frosting evenly over the cooled sugar cookie crust using a spatula.
08 - Place blueberries in the top-left corner for the 'stars.' Layer sliced strawberries and raspberries in stripes, alternating with the frosting to represent the flag pattern.
09 - Refrigerate for at least 30 minutes prior to slicing and serving for best texture and presentation.