In the bowl of a stand mixer, whisk together egg whites and sugar. Place over boiling water and whisk until hot, about 3 minutes.
Place on stand mixer, and, with the whisk attachment, whip on medium-high until cool, about 5 minutes.
Switch to the paddle attachment, and, on medium-high, add the butter 4 tablespoons at a time, allowing it to become fully incorporated before adding the next.
Mix on medium-high until thick and smooth, 6 to 10 minutes.
Turn mixer to medium, and add vanilla and any other flavorings or colorings.
