Combine egg whites, sugar, and cream of tartar in a heat-proof bowl, ideally your mixing bowl.
Place over top of a large saucepan of simmering water, and whisk constantly for about 2 minutes, or until the sugar has dissolved and the egg whites are warm to the touch.
Immediately remove the bowl from heat, and place in your stand mixer. Start beating the egg white mixture slowly at first, but gradually increase speed to high.
Beat for 6-7 minutes, or until glossy and thick.
Add vanilla, and beat until combined.
Frost cupcakes.