In the metal stand mixer bowl set over a saucepan of simmering water, whisk the egg whites with the sugar until the whites are warm and the sugar is dissolved, about 3 minutes until hot to touch. Take the bowl off of the water and attach it to the stand mixer fitted with the whisk. Add the vanilla and cream of tartar and beat at medium speed until firm. Increase the speed to high and beat the meringue until stiff and glossy, 5 to 7 minutes. Spoon mixture over the brownies. With a torch, lightly brown the top of the meringue. Cut into squares and serve.