In a large mixing bowl or the bowl of a stand mixer, mix the butter on high speed until light and fluffy.

Add the confectioner’s sugar in small increments, alternating with the milk and mixing well after each addition. Use a rubber spatula to scrape down the sides of the bowl as needed, until you’ve fully incorporated all of the sugar.

Add the vanilla extract and mix until smooth and creamy, about 3-5 minutes. If necessary, add more confectioner’s sugar or milk to adjust the consistency. The buttercream should be thick and creamy, yet soft enough to pipe.

Portion the buttercream into six individual bowls.

Add 2-3 drops of gel food coloring to each bowl.

Stir to create six differently-colored buttercreams: red, orange, yellow, green, blue, and purple.

Transfer each color to its own disposable piping bag. Use kitchen shears to snip the tip of each bag.

Unroll a piece of clingfilm/plastic wrap, about 16-18 inches long. Pipe a stripe of each color on top of the plastic, mimicking the color pattern of the rainbow. Use all of the buttercream.*

Gently roll the clingfilm around the buttercream, forming a tube.

Cut the tube in half.**

Place one half of the tube, cut side facing down, inside a larger piping bag fitted with a star, French star, or other piping tip of choice.

Pipe as desired.

Repeat with a clean piping bag and the second half of the buttercream tube.