01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - In a medium bowl, whisk together flour, baking powder, and salt; set aside.
03 - In a large bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Mix in vanilla.
04 - Add half the dry ingredients to the butter mixture, then milk, then the remaining dry ingredients; mix just until combined.
05 - Divide batter evenly into three bowls. Tint one portion red, one blue, and leave one portion plain.
06 - Using a spoon, layer each color batter into liners in a random or swirled pattern for a tie dye effect.
07 - Bake for 16–18 minutes, or until a toothpick inserted in the center comes out clean. Cool completely on a wire rack.
08 - For buttercream, beat butter until creamy. Gradually add powdered sugar, milk, vanilla, and salt until light and fluffy. Divide into three bowls; tint one red, one blue, and leave one white.
09 - Fill a piping bag with all three colors side-by-side to create a swirl. Pipe buttercream onto cooled cupcakes.
10 - Optionally, decorate with festive sprinkles.