2 large egg whites
1/2 cup sugar
1/2 teaspoon vanilla extract
Finely grated zest of 1/2 lemon
3 tablespoons unsalted butter, chilled and cut into cubes
Here’s what you do:
Combine the egg whites, sugar, salt and vanilla extract in a heatproof bowl and set over (not in) a pan of simmering water. Stir continuously over heat until the mixture is warm and the sugar is completely dissolved, 1 to 2 minutes.
Remove the bowl from the heat.
Using a handheld electric or standing mixer, set on high beat until the mixture is entirely cooled, glossy and stiff. Beat in the lemon zest, and add the butter cubes, a few at a time, beating until the butter is almost fully incorporated after each addition. Beat until smooth. Let frosting set and cool in the refrigerator (at least 30 minutes) before applying to cake.