In the bowl of a stand mixer, combine egg whites, sugar and salt. Set the bowl over simmering water and whisk until sugar has dissolved. Takes 2-3 minutes. You want the mixture to be hot and not feel gritty between your fingers
Transfer to a stand mixer with whisk attachment. start on low speed and gradually increasing to med/high during the first minute, Beat mixture until stiff glossy peaks form. This will take 7-10 minutes.
Reduce speed to low/med and gradually add tablespoons of butter. Beating well between each addition. Don't worry if it looks curdled but just keep adding butter, it will become smooth.
Add vanilla and beat until combined
At this stage, switch to paddle attachment for 2 minutes to remove air bubbles
Use immediately, makes about 5 cups