Fill a medium to large pot with 1-inch of water and bring to a simmer.
In a large heat-proof bowl (or the bowl of your stand mixer), whisk together egg whites, sugar, cream of tartar and salt.
Place bowl over the simmering water on the stove, making sure the bottom of the bowl is not touching the water.
Heat, whisking constantly until eggs reach 120°F.
Remove bowl from heat and use a hand mixer or electric mixer to beat the egg whites on high for 5 minutes or until light and fluffy with peaks that mostly hold their shape.
Add vanilla extract and beat for another 30 seconds until well-mixed.
Transfer immediately to a piping bag or use a large plastic bag with the tip cut off to pipe onto cupcakes.
Very carefully brown under your broiler or toast with a kitchen torch; this step is optional, but the frosting is quite sticky until toasted.
