• Step 1: Preheat the oven to 350°F. Grease and flour three 8-inch round baking pans.
• Step 2: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
• Step 3: In a large bowl, beat the butter and sugar together until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time, then stir in the vanilla.
• Step 4: Alternate adding the flour mixture and milk to the butter mixture, mixing just until incorporated. Be careful not to overmix.
• Step 5: Divide the batter evenly into three bowls. Color one bowl red with food coloring and one bowl blue. Leave the third bowl plain for the white layer.
• Step 6: Spread the colored batters into the prepared pans, creating alternating red, white, and blue layers.
• Step 7: Bake for 18-22 minutes, until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
• Step 8: Spread the vanilla buttercream frosting evenly over the top and sides of the cooled cake, creating a smooth finish.