01 - Preheat oven to 350°F. Line a large baking sheet with parchment paper or use a 9x13-inch baking pan.
02 - In a large mixing bowl, beat butter and granulated sugar until light and fluffy. Add egg and vanilla extract, mixing thoroughly.
03 - In a separate bowl, whisk together flour, baking powder, and salt. Gradually add these dry ingredients to the wet mixture, combining until a soft dough forms.
04 - Press the dough evenly into the prepared pan, creating a uniform 1/4-inch thick crust. Bake for 13 to 15 minutes until the edges are lightly golden. Allow crust to cool completely in the pan.
05 - In a clean bowl, beat cream cheese and butter until smooth. Add powdered sugar and vanilla extract, mixing until creamy and spreadable.
06 - Evenly spread the cream cheese frosting over the cooled sugar cookie crust, ensuring complete coverage to the edges.
07 - Decorate by arranging blueberries in the top left corner. Create red stripes with strawberries and raspberries, alternating with the white frosting to resemble a flag.
08 - Refrigerate the assembled dessert for at least 30 minutes before slicing and serving for best results.