Prepare a cupcake pan with cupcake liners and preheat the oven to 350°F.

Combine the flour, baking powder, and salt in a medium-sized bowl and set aside.

Add the butter, oil, and sugar to a large mixer bowl and beat until light and fluffy, about 2-3 minutes.

Add the eggs one at a time, mixing until mostly combined after each, then add the vanilla extract with the second egg.

Scrape down the sides of the bowl as needed.

Add half of the dry ingredients and mix until mostly combined.

Slowly add the buttermilk and mix until well combined; the batter might look slightly curdled.

Add the remaining dry ingredients and mix until the batter is smooth.

Divide the batter into three bowls: leave one as-is for white, color the second red, and the third blue.

Gently fold the batter to avoid overmixing.

Fill each cupcake liner with about four tablespoons of batter in two layers with different colors.

Bake for 15-20 minutes or until a toothpick inserted in the center comes out clean.

Remove and allow cooling on a rack.

To make the buttercream, beat the butter until smooth, then add half the powdered sugar and mix until smooth.

Add the vanilla extract and one tablespoon of water or cream; mix until smooth.

Add remaining powdered sugar and mix until smooth.

Adjust consistency with more water or cream, then add salt to taste.

Divide the buttercream into three bowls; leave one as-is, color the second red, and the third blue.

Make a swirl frosting using a ziplock bag and clear wrap to create a piping log.

Pipe swirls of frosting onto the cooled cupcakes and serve.