Combine egg whites, sugar, and salt in a bowl and set over a pan of simmering water.

Cook until the sugar is melted and warm.  You do not want to feel any of the sugar grain when you rub the mixture between your fingers.

Beat the egg/sugar mixture on high speed with whisk attachment until soft peaks form and the mixture cools down.  Switch to paddle attachment if you have one.

Add butter a few tablespoons at time until all the butter is incorporated.  Add the vanilla and beat until smooth.