01 - Preheat oven to 350°F. Line a 9 x 13 inch baking pan with parchment paper and lightly grease.
02 - In a large bowl, beat together softened butter and granulated sugar until light and fluffy, about 2 to 3 minutes.
03 - Incorporate eggs one at a time, beating well after each addition. Mix in the vanilla extract.
04 - In a separate bowl, whisk together all-purpose flour, baking powder, and salt.
05 - Gradually mix dry ingredients into the butter mixture until just combined.
06 - Evenly press the dough into the prepared baking pan and smooth the surface with a spatula.
07 - Bake for 18 to 22 minutes, until the edges turn lightly golden and the center sets. Avoid overbaking.
08 - Allow to cool completely in the pan on a wire rack.
09 - Beat softened butter until creamy. Gradually blend in powdered sugar, then mix in vanilla extract and heavy cream. Whip on high for about 2 minutes until light and fluffy.
10 - Divide the frosting into three bowls. Leave one plain, tint one bowl red, and one bowl blue with gel food coloring.
11 - Dollop spoonfuls of each colored frosting over cooled bars. Gently swirl with an offset spatula for a marbled effect or frost in sections for striped appearance.
12 - Immediately sprinkle red, white, and blue sprinkles over frosting.
13 - Cut into 24 bars and serve.