Before you begin: prepare a double boiler. I like to use my stainless steel KitchenAid mixing bowl (since I like to do all the mixing with my stand mixer) and a medium-sized saucepan that is large enough to suspend the bowl without it touching the bottom of the pan. Make sure that the bowl that you will be adding your egg whites to is completely clean, dry, and grease-free.

In the bowl that you are using for your double boiler (I use my stand mixer bowl), combine sugar, corn syrup, egg whites, cream of tartar, and salt and whisk until well-combined.

Bring 1-2” of water to a simmer over medium-low heat in the bottom of your double boiler (Note: I use a medium-sized saucepan. Make sure that when the bowl of your double boiler is added to the pot it does not touch the water).

Place the bowl with egg whites over simmering water and whisk constantly until the sugar is dissolved and temperature of mixture reaches at least 160F (70C). Note: if you rub a small amount of the syrup between your fingers, it should feel smooth and not at all gritty. Be careful though, the syrup is hot!

Remove bowl from heat and fit it onto your stand mixer (fitted with a whisk attachment). Slowly increase mixer speed to high and beat on high speed for about 5-10 minutes, until you have thick, billowy, glossy stiff peaks.

Stir in vanilla extract until completely combined.

Use marshmallow frosting immediately or cover tightly and store at room temperature until ready to use (you may need to re-whip the frosting before using if it sits longer than several hours). If desired, toast frosting using a culinary torch after piping and before serving.