This American flag cake is a real showstopper. The base is a buttery shortbread cookie that's spread with a tangy cream cheese frosting. It's then decorated with raspberries, white chocolate chips and blueberries for the iconic red, white and blue.
For the shortbread: Preheat the oven to 400 degrees F.
In a large bowl, whisk together the flour and brown sugar. Cut the butter into pieces, and using 2 forks or your fingers, work the butter into the flour mixture until it's the consistency of wet sand. Press the dough firmly and evenly into a nonstick 9-by-13-inch rimmed baking sheet.
Bake until the shortbread is pale golden, 10 to 15 minutes. Remove from the oven and let cool completely in the baking sheet on a wire rack. Once it has cooled, use a sharp knife to carefully cut around the edges then invert the shortbread onto a serving platter.
For the toppings: Make the frosting by beating the cream cheese in the bowl of a stand mixer fitted with the paddle attachment on low speed until smooth, about 2 minutes. Add the butter and continue beating on low speed until combined, about 2 minutes. Add the vanilla extract then sift the confectioners' sugar into the bowl and mix on low speed just until combined. (Overmixing will cause the frosting to be too runny.)
Spread the cream cheese frosting in an even layer on the shortbread. Arrange 6 of the blueberries in the top left corner in each of 5 rows to form the blue portion of the flag. Arrange the raspberries in about 6 horizontal rows across the cake, leaving about 1 inch between each row, to form the stripes. Place the white chocolate chips between the blueberries to form the stars.
Using a sharp knife, slice and serve the cookie cake immediately, or refrigerate until ready to serve the same day. To make the cake in advance, wrap the shortbread tightly in plastic wrap and keep at room temperature for up to 2 days. Refrigerate the frosting for up to 2 days. Then assemble the cake shortly before serving so the berries don't release juices into the frosting.