Preheat your oven to 350°F (175°C) and prepare two 8-inch square baking pans by lining them with parchment paper.

Cream softened unsalted butter and granulated sugar in a stand mixer until light and fluffy, then add egg whites and vanilla extract.

Whisk together salt and baking powder with half of the cake flour, then alternately add this dry mixture and milk to the creamed mixture.

Divide batter into three bowls; color one with Americolor Super Red and another with Americolor Royal Blue, leaving one plain.

Spoon dollops of each colored batter into prepared pans and swirl gently using a knife.

Bake for 30-32 minutes until a toothpick inserted comes out clean; monitor closely to avoid overbaking.

Cool the cakes in pans for 10 minutes, then transfer to wire racks to cool completely and slice each cake in half.

Beat together unsalted butter and shortening for frosting, then gradually add powdered sugar, heavy cream, and vanilla extract.

Stack cake layers with frosting between each, adding a crumb coat around the outside and chill for 30 minutes.

Decorate the chilled cake with remaining frosting and sprinkle with non-pareils if desired.