Weigh out your egg whites in the bowl of your mixer. Sift your confectioners sugar into the bowl.
Make sure your mixer has the whisk attachment on. Begin mixing it on low, slowly increasing it to high and let it go for about 6 minutes.
Change out your whisk attachment for the paddle.
Turn the mixer on low and add your butter in 1 stick at a time.
Add your salt and flavoring and let the mixer go on low for about 10 minutes.
Cake batter extract is usually bright yellow and may change the color of your buttercream. To neutralize the yellow and bring it closer to white, you can add 1 small drop of purple gel food coloring.
Mix your sprinkles in with a spatula if you're using them.