01 - Preheat the oven to 350°F. Line a baking sheet with parchment paper or lightly grease a 9x13-inch pan.
02 - In a large bowl, beat unsalted butter with granulated sugar until light and fluffy. Blend in the egg and vanilla extract until incorporated.
03 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt. Gradually mix the dry mixture into the wet ingredients to form a soft dough.
04 - Press dough evenly into the prepared pan to an even 1/4-inch thickness. Bake 13 to 15 minutes or until edges are lightly golden. Cool completely in pan.
05 - In a medium bowl, beat softened cream cheese and unsalted butter together until smooth. Add powdered sugar and vanilla extract; mix until creamy and spreadable.
06 - Spread the cream cheese frosting evenly over the cooled sugar cookie crust.
07 - Arrange blueberries in the upper-left corner to symbolize stars, then alternate stripes of strawberries and raspberries lengthwise in red stripes, using white frosting spaces for contrast.
08 - Chill assembled dessert for at least 30 minutes before slicing and serving for cleaner cuts.