In a heatproof bowl, combine the egg whites and sugar.
Place the bowl over a saucepan of simmering water, making sure the bottom of the bowl does not touch the water.
Whisk continuously until the sugar has dissolved, and the mixture is hot to the touch.
Remove the bowl from the heat and whisk on high speed until the mixture forms stiff peaks.
Reduce the speed to low and gradually add the butter, a few tablespoons at a time, until all the butter is incorporated.
Continue to whisk until the buttercream is smooth and creamy.
Use immediately or refrigerate until ready to use.
