White Mountain Frosting

Step 1
Stir sugar, water, and cream of tartar together in a saucepan over medium heat. Cover and bring to a boil, then uncover and cook until a candy thermometer reaches 238 degrees F (115 degrees C), 5 to 7 minutes; syrup should spin a long thread when dripped from a spoon.

Step 2
While the syrup is cooking, beat egg whites in the bowl of a stand mixer with the whisk attachment until stiff peaks form.

Step 3
When the syrup has reached temperature, slowly pour a thin stream into egg whites, beating constantly, until frosting stands in peaks. Stir in vanilla and salt.