Prepare the cake mix batter following the directions on the box. Line two 12-cup muffin pans with cupcake liners; this recipe yields about 20 cupcakes—if you stretch it to 24, they tend to turn out smaller than ideal. Bake as instructed, then allow the cupcakes to cool completely before frosting.

Using a hand or stand mixer, beat the softened butter on medium speed for about 3 minutes until it becomes smooth and creamy. With the mixer on low speed, gradually add the powdered sugar, heavy cream, and clear vanilla extract. Once incorporated, increase to high speed and beat for another 3 minutes until fluffy. You can use the frosting right away or refrigerate it in an airtight container for up to two weeks.

Tip: If your frosting feels too thick, add a tablespoon of cream at a time, mixing thoroughly after each addition, until it reaches the desired consistency.

Divide the buttercream evenly into six medium bowls—cereal bowl size works well. Tint each portion with your chosen gel food coloring. For vibrant hues, use high-quality food coloring gels (like those available online). Grocery store liquid food coloring tends to yield more muted shades, which won’t give the bold, rainbow effect seen in photos.

Transfer each colored frosting into a separate tipless piping bag and snip a small corner off each one. On a large sheet of plastic wrap laid out on your counter, pipe parallel lines of each color. Carefully roll the plastic wrap into a tight log (similar to a cigar) and twist the ends to secure. Snip off one end of the plastic-wrapped frosting log and insert it into a large piping bag fitted with a 1M star tip. Pipe beautiful rainbow swirls onto the cooled cupcakes. These cupcakes can be served immediately or stored in the refrigerator (covered) for up to one week, although like most baked goods, they taste best on the first day.