In a small heatproof bowl, whisk the sugar and egg whites. In a 1-quart saucepan, bring 2 cups of water to a simmer over medium-high heat. Set the bowl over the simmering water (don’t let the bowl touch the water) and heat the mixture, whisking occasionally, until hot to the touch, 4 to 6 minutes. It will thin out a bit as the sugar melts.
Remove the bowl from the heat and scrape the mixture into the bowl of a stand mixer fitted with the whisk attachment. Whip on medium-high speed until the mixture is light, white, and cool to the touch, 4 to 6 minutes. Reduce the speed to low and add the chunks of butter one at a time. Increase the mixer speed to medium and beat until the buttercream is smooth, 4 to 5 minutes. Mix in the vanilla and salt.
Make Ahead Tips
The buttercream can be refrigerated in an airtight container for up to 4 days. Before using, return to room temperature and then mix the buttercream in a stand mixer fitted with the paddle attachment on medium-low speed until smooth, 1 to 2 minutes.
Espresso Buttercream: Substitute 2 tsp. instant espresso powder dissolved in 1/2 tsp. warm water, cooled, for the vanilla and mix it in along with the salt.
Photo: Colin Clark