In a dry, grease-free, heatproof mixing bowl (or in the bowl of a stand mixer), whisk together egg whites and sugar. Set bowl over a double boiler (saucepan filled with a few inches of simmering water) over medium heat. Make sure that the bowl does not touch the water. Whisk the whites and sugar constantly until sugar is dissolved and mixture has thinned out, or until mixture reaches 160°F, about 4 minutes.
When the mixture is still warm, transfer to the bowl of stand mixer fitted with a whisk attachment. On high speed, beat the mixture until stiff glossy peaks form and the meringue is no longer warm to the touch, at least 10-15 minutes. If it still feels warm, wait until room temperature before adding butter.
On medium-high speed, add the butter 1 tablespoon at a time. Wait for the butter to fully mix in before adding the next tablespoon. After all the butter has been added, turn the mixer down to medium speed and beat until thick and creamy, around 2-4 minutes more. Finally, fully beat in the vanilla and pinch of salt, about 30 seconds.