Fill a medium pot with a few inches of water and bring to a simmer.

In the bowl of a stand mixer, combine the egg whites, sugar, and salt. Place the bowl over the pot of simmering water and whisk constantly to dissolve the sugar, about 5 minutes. Feel the mixture between your fingers. If you feel granules of sugar, continue to whisk until it feels smooth.

Remove from heat and place in the mixer fitted with the whisk attachment. Turn the mixer on high and whip until meringue is thick and glossy and bowl is cool to the touch, about 5 minutes.

With the mixer on medium-high, add in the butter, a tablespoon at a time until the butter is fully incorporated and the buttercream is smooth and glossy. Switch to the paddle attachment and add in the vanilla and whip to combine.