Place egg white, salt and sugar in a heat-proof bowl and cook over a double boiler, stirring with a hand whisk at all times until sugar has completely dissolved.

Transfer the egg white mixture into the stand mixer bowl and using the whisk attachment, beat with high speed until stiff peaks and the mixture has cooled to room temperature. If they’re still hot to the touch, even if they look white and stiff, they will melt the butter when you add it.

Once the egg whites are room temperature and stiff, add the butter a cube at a time, while the mixer runs on medium speed. It’s important to add it slowly, to give everything time to emulsify properly.

Once all of the butter is added, increase the speed again and whip until it comes together and is light and fluffy. Scrape down the bottom and sides of the bowl every once a while.

You can store the buttercream in the freezer up to one month. When you want to use it, bring it to room temperature then whip on low speed until smooth again.