1 c egg whites (from about 6 large eggs)1 1/2 c sugarpinch of salt1 lb 4 oz unsalted butter, at room temperature2 t vanilla extract (or seeds of 1/2 vanilla bean)squeeze of lemon juice
1)Combine the egg whites, sugar and pinch of salt in a large metal bowl (I usually do this right in my stand mixer bowl). Put the bowl over a pot of simmering water (double boiler-style), and whisk until the sugar is completely dissolved. The mixture will feel hot to the touch.
2)Transfer the hot mixture to your stand mixer and whip on high speed until it turns white and about doubles in size. This will take about six minutes. Beat in the vanilla and the lemon juice.
3)Add the butter, a few tablespoons at a time, on medium speed, mixing after each addition. Raise the speed and beat until smooth; this may take up to ten minutes. The mixture may appear curdled along the way; this is normal. Just keep beating and the mixture will become smooth again.
-Buttercream will keep, covered air-tight and refrigerated, for up to two weeks. Bring to room temperature, then beat on low speed before using.