If you have a scale, weigh the butter and powdered sugar. If you don’t have a scale, use the measurements listed in the recipe.
Ensure that the butter is softened, and at room temperature.

Add the boiling water to the powdered sugar and vanilla extract and whisk until dissolved.

Pass the sugar solution through a fine sieve to remove any lumps. You should have a little over a cup of liquid, as shown.
Let the solution cool completely.

Add the cooled sugar solution to the butter while the KitchenAid mixer is running. Add it slowly and allow it to incorporate a little at a time.

If you like, you can add food coloring to the whole batch, or you can split the batch into two portions, and add different colors to each. Mix well.

To make a two-color swirl, spoon two thick lines of frosting onto a large sheet of plastic wrap. Roll the plastic into a frosting-filled sausage. 😄 Twist the ends to seal.

Cut one end of the plastic off and place the frosting sausage into a piping bag fitted with your tip of choice.

Pipe the frosting onto the cupcake or cake in a swirl. 🧁