Preheat oven to 350 degrees F. Line a muffin tin with cupcake liners.
Once you have the white cake mix batter made, you'll need to separate it so that you can create the red batter and blue batter. Pour about 1 cup of the plain batter into 2 small bowls (1 cup of batter for each bowl.)
I used about 1/8th teaspoon of coloring in each bowl, mixed to combine.
Scoop 1 heaping tablespoon of white batter into the bottom of each cupcake. Then scoop about 1 level tablespoon of red batter onto one side of each cupcake, followed by the same amount of the blue batter on the other side. Add 1 more level tablespoon of white batter to the top of each cupcake. (Cupcake liners are pretty much full at this point!) Take a toothpick and gently swirl the batter in each cupcake around a few times.
Bake according to package directions- mine baked for 18 minutes.
Cool completely and frost with my favorite frosting- recipe is included!
Cream the butter until it is light and fluffy. Add the marshmallow creme.
Whip for 2 minutes.
Add the powdered sugar and continue to beat for 2 minutes. Add vanilla extract and milk until it’s the texture that you want. If you’re frosting cupcakes, a slightly thicker frosting is better because they hold their shape. However if you’re frosting a cake, I’d add more milk- 3 TBSP or even 4, so that you can spread the frosting well.