Instructions

• Fill a medium-to-large pot with about 1 inch (2.5 cm) of water and bring it to a gentle simmer over medium heat. Choose a heat-proof bowl (or the bowl of your stand mixer) that will sit on the pot without the bottom touching the water.
• In the heat-proof bowl, whisk together 4 large egg whites (room temperature), 1 cup (200 g) granulated sugar, 1/4 teaspoon cream of tartar, and a pinch of salt until evenly combined.
• Set the bowl over the simmering water (double-boiler setup), making sure the bowl does not touch the water. Whisk constantly until the mixture reaches 120°F (49°C) on an instant-read thermometer and the sugar feels dissolved (rub a small amount between your fingers—it should not feel gritty).
• Remove the bowl from the heat. Using a stand mixer fitted with the whisk attachment or a hand mixer, beat the hot mixture on high for about 5 minutes, or until it is light, glossy, and fluffy with peaks that mostly hold their shape.
• If using, add 1/2 teaspoon vanilla extract and beat on high for another 30 seconds to incorporate.
• Use the frosting immediately: transfer to a piping bag or a large plastic bag with the tip cut off and pipe onto cupcakes or cake.
• If you want a toasted finish, very carefully brown the frosting under a hot broiler for a few seconds (watch continuously) or use a kitchen torch. The frosting will be quite sticky until it is toasted.