Swiss Meringue Buttercream
2 egg whites
1/2 cup plus 2 tbsp. sugar
1/2 c. (1 stick) unsalted butter, cut into small pieces and softened
1/2 tsp. pure vanilla extract
Whisk together egg whites and sugar in a heatproof mixer bowl. Set bowl over a pan of simmering water, whisking until sugar is dissolved and mixture is hot (140-160 degrees).
Using an electric mixer fitted with the whisk attachment, beat mixture on medium-high speed until stiff peaks form, and the bottom of the bowl comes to room temperature, about 10 minutes.The meringue will be white with a bright sheen and very sticky!
Remove whisk attachment and replace with paddle attachment. On medium-low speed, add butter in small additions until incorporated. Switch to the paddle attachment, and on medium speed add the butter a few cubes at a time. Please note, if the meringue is not completely cool, the butter will melt instead of combining evenly. At one point it may appear as if the mixture has separated like curdled milk. Continue mixing until fluffy.
Reduce speed to low; add vanilla, mixing until combined.
Scrape down sides of bowl, and continue mixing on medium speed until buttercream is smooth and creamy.