Preheat oven to 350°F (175°C). Line a 9×13-inch pan with parchment paper.
Melt butter in a large saucepan over low heat. Remove from heat and whisk in sugar. Add eggs one at a time, whisking after each. Stir in vanilla.
Sift in flour, cocoa, salt, and baking powder. Stir until *just* combined — stop as soon as the flour disappears.
Pour into prepared pan and spread evenly. Bake 25–30 minutes, until the edges are set and the center has a very slight jiggle. Do not overbake.
Cool completely in the pan (at least 2 hours or overnight) before decorating.
Decorate: Divide the white buttercream into three portions. Leave one white, tint one red, one blue (use gel food coloring for vivid color). Frost each brownie with a white base. Pipe red and white horizontal stripes across the bottom three-quarters. Add a blue rectangle in the upper left corner and press in star sprinkles.
