Make the Cupcakes: Preheat oven to 350°F (175°C) and spray a muffin tin with cooking spray or line with cupcake liners.

Prepare Batter: Prepare the cake batter according to the package instructions.

Divide and Color Batter: Divide the batter among five bowls. Add a few drops of red food coloring to one bowl, yellow to the second, green to the third, blue to the fourth, and two drops of blue plus two drops of red to the fifth (to create purple). Stir each bowl, adjusting the amount of food coloring you want until you have five colors of cake batter.

Layer Batter in Liners: Spoon a little of each color of batter into each cup of the muffin tin, until they’re filled two-thirds of the way up. This creates the rainbow swirl effect within the cupcakes.

Bake: Bake according to the package instructions, or until a toothpick has moist crumbs.

Cool: Set the cupcakes aside to cool completely.

Make the Frosting: Meanwhile, make the frosting. In a large bowl using a hand mixer or in the bowl of a stand mixer using the whisk attachment, beat the softened butter until fluffy. Add the powdered sugar and beat until combined, then add the vanilla extract, heavy cream, and salt and beat until combined.

Divide and Color Frosting: Divide the frosting into five bowls, adding food coloring to each and stirring, just like you did with the cake batter (red, yellow, green, blue, violet/purple).

Prepare Piping Bag: Place each color of frosting in a resealable sandwich bag and snip off one corner.

Create Frosting Tube: Lay out a piece of plastic wrap. Use each bag to pipe out a line of frosting in each color, side by side, on the plastic wrap. Roll up the plastic wrap, creating a tube of frosting. Twist the ends closed.

Place in Piping Bag: Slip a star tip onto one end of the rolled-up frosting tube. Trim the plastic wrap that sticks through the star tip. Place the entire tube inside a large piping bag.

Frost Cupcakes: Pipe the frosting onto the cooled cupcakes.

Serve