Preheat the oven to 350°F and line a muffin tin with cupcake liners.

Cream the softened butter and sugar together in a bowl until light and fluffy. Beat in the egg and vanilla extract until smooth.

Gradually add the flour and baking powder, stirring gently to combine. Mix in the milk until the batter is smooth.

Divide the batter evenly among the cupcake liners, filling each about halfway. Bake for 12-15 minutes, or until a toothpick inserted into the center comes out clean. Let the cupcakes cool completely on a wire rack.

For the icing, beat the softened butter until creamy. Slowly add the powdered sugar, mixing until smooth. Add a small splash of milk if needed to achieve your desired consistency.

Divide the icing into three bowls and tint each with a drop of food coloring, mixing until the colors are even. Transfer each color into a piping bag with a star nozzle.

Once the cupcakes are fully cooled, pipe the icing onto each one in a swirl pattern. Finish with sprinkles for a festive touch.