This buttercream is a lot easier to make that its name leads us to believe, and well worth the work.
5 large egg whites
1 cup sugar
a pinch of salt (except if you use salted butter)
1 lb butter (I do use salted butter...!) at room temperature
1 Tbsp vanilla extract (I used homemade)
In the bowl of a stand mixer, whisk together the egg whites, sugar and salt. Set over a pan of simmering water and whisk constantly until the sugar has dissolved.
Fit the stand mixer with the whisk and start to slowly whisk the egg whites mixture. Gradually increase the speed and whisk until stiff peaks form. Continue whisking until it becomes glossy and completely cool, about 10 minutes.
With the mixer at low speed, add the butter, a couple of tablespoons at a time. When all the butter has been incorporated, add the vanilla extract. Switch from the whisk to the paddle attachment and mix on low speed until all there are no more air bubbles.
You can keep for up to 3 days in the fridge, or a month in the freezer, in an airtight container.
Bring to room temperature before using.