01 - Preheat the oven to 350°F. Line a 9-by-13-inch baking pan or large baking sheet with parchment paper.
02 - In a large bowl, beat softened butter and granulated sugar with an electric mixer until the mixture is light and fluffy. Add in the egg and vanilla extract, continuing to mix until fully incorporated.
03 - In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture and mix just until a soft dough forms.
04 - Press the dough evenly into the prepared pan, forming a crust approximately 1/4 inch thick. Bake for 13 to 15 minutes or until the edges develop a light golden color. Remove from the oven and allow the crust to cool completely in the pan.
05 - Using a clean mixing bowl, beat the softened cream cheese and butter until smooth and creamy. Add the powdered sugar and vanilla extract. Continue beating until the frosting is well-blended, smooth, and spreadable.
06 - Spread the cream cheese frosting evenly over the cooled sugar cookie crust using a spatula. In the upper left corner, arrange the blueberries to represent the field of stars. Create alternating red stripes across the rest of the surface with sliced strawberries and raspberries, leaving white frosting visible to mimic the flag pattern.
07 - Refrigerate assembled dessert pizza for at least 30 minutes before slicing and serving for best texture and presentation.