Makes enough to frost 15-18 cupcakes generously.
4 ounces (1/2 cup) egg whites
8 ounces (1 cup) granulated sugar
8 ounces (2 sticks) unsalted butter, at room temperature
1/2 teaspoon vanilla extract
Mix egg whites and sugar together in a heat-proof bowl.
Place bowl on top of a boiling water bath and whisk until frothy (about 240 F). Coat the back of a spoon with the mixture to double check that all granules of sugar are completely dissolved. Allow mixture to cool before continuing.
Transfer mixture to the stand mixer bowl with a whip attachment. Whip on high until stiff peaks form (about 5-8 minutes).
Add butter, a tablespoon at a time, into the mixture while whipping on high. The frosting will deflate and then eventually look broken. Do not fear. Continue mixing and adding butter a tablespoon at at time until the frosting gets smooth and fluffy.
Add vanilla extract. Mix until just combined.