Put an inch of water in a medium saucepan and bring to a gentle boil. In the bowl of your stand mixer, stir egg whites, sugar and salt until combined.
Place mixing bowl over saucepan but be careful not to let the water touch the bottom of the mixing bowl. Stir with a spatula until the sugar dissolves and reaches a temp of 160 F, about 5 minutes. While you stir, scrap the sides of the bowl to ensure no sugar grains are lurking on the sides.
Remove mixing bowl from heat and add to stand mixer with whisk attachment. Whisk on medium high speed until stiff peaks form. They will be glossy. This can take up to 10 minutes. The bowl will continue to cool down in the process as well.
Reduce the speed to low and add 1 to 2 tablespoons of butter at a time making sure to mix in thoroughly after each addition. When butter is incorporated, add vanilla extract and beat on low until incorporated then frost or refrigerate until ready to frost.
During the double-boiling process, it's crucial that the sugar completely melts and combines with the egg whites. To test, dip a finger into the mixture and rub it together with your thumb. If you feel any grit, keep cooking.