makes enough to frost a double layer cake or two dozen cupcakes
Whisk egg whites until foamy, using whisk attachment on a stand mixer. Sprinkle in the cream of tartar, vanilla, pinch of salt, then gradually increase speed to high and whisk until egg whites hold soft peaks. Turn the speed to low as you make the sugar syrup (next step).
Combine the sugar and water in a small saucepan and set on the stovetop over medium heat. Cook, without stirring, until sugar is dissolved and temperature reaches soft-ball stage on a candy thermometer (225°–230°F at 5000 ft. elevation). Slowly dribble the boiling sugar syrup into the egg whites while the mixer is running on low, doing your best to aim the syrup between the whisk and the side of the bowl for best incorporation. When all the syrup has been added, increase mixer speed to medium-high and beat until mixer bowl is cool and egg whites form "stiff, shining, upstanding peaks" (thanks Julia Child!) which will take about 8 minutes. Use immediately to frost a cake, top a meringue tart, or make meringue cookies.