Place egg whites, sugar, and cream of tartar in a heatproof bowl. Set bowl over a saucepan filled with two inches of simmering water. Do not let it touch the water. (You can use a double boiler if you have one.)

Whisk constantly until sugar is dissolved and mixture has thinned out, about 4 minutes. The mixture will be thick and tacky at first, then thin out and appear frothy on top. To test that it’s ready, you can use your finger or an instant read thermometer. Lightly and quickly dip your finger (it’s very hot, be careful) and rub the mixture between your thumb and finger. You shouldn’t feel any sugar granules. If using a thermometer, the temperature should read 160°F (71°C).

Remove from heat. (No need to let it cool down before continuing.) Add the vanilla extract, then using a handheld or stand mixer fitted with a whisk attachment, beat on high speed until stiff glossy peaks form, about 5 minutes.

Meringue can be spread, piped, or swirled onto cakes, cupcakes, and other confections. Serve immediately OR torch it with a kitchen torch for a delicious toasted marshmallow topping. (Do not place in the oven under the broiler– it will melt.)

Cover and store leftovers for up to 2 days in the refrigerator. Baked goods topped with this marshmallow meringue can be left at room temperature for up to 6-8 hours. After that, it’s best to refrigerate or else the topping will begin to wilt. For best taste, texture, and appearance, I do not recommend freezing this.