Place egg whites, sugar, and cream of tartar in the heatproof bowl of an electric mixer. Set over a saucepan with simmering water. Whisk constantly until sugar is dissolved and whites are warm to the touch, 3 to 4 minutes.
Transfer bowl to electric mixer fitted with the whisk attachment, and beat, starting on low speed, gradually increasing to high, until stiff, glossy peaks form, 5 to 7 minutes. Add vanilla, and mix until combined. Use immediately.
Transfer frosting to a large pastry bag fitted with a large plain round or French tip. Pipe frosting in a spiral motion on each cupcake. Transfer cupcakes to a baking sheet. Using a kitchen torch, lightly brown the frosting, taking care not to burn the cupcake liners. Serve immediately or store in an airtight container, up to 2 days.