Prepare an electric mixer by fitting it with the whip attachment. Bring a medium pot filled with a few inches of water to a gentle simmer.
Combine the egg whites and sugar in the bowl of an electric mixer, and place over the pot of water. Heat, stirring constantly so the eggs don’t scramble, until the mixture reads 160° F on a thermometer.
When the egg and sugar mixture has come to temperature, transfer the bowl to the mixer. Whip on high speed until the mixture reaches stiff peaks and the bowl is no longer noticeably warm to the touch. The meringue should be smooth and glossy, not clumpy or dry.
While the mixer is running, gradually add room temperature butter in 1/2-tablespoon chunks. Continue adding and mixing until all of the butter is incorporated and the buttercream is light and smooth. If the buttercream looks curdled or separated, continue whipping for a few minutes—it should come together.
Beat in the vanilla and mix to combine. The buttercream can be used immediately or refrigerated in an airtight container.
I always carry three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's pie. My first cookbook, The Fearless Baker, is out on October 24, 2017.