Seven-minute Frosting
from https://www.kingarthurbaking.com/

Seven-minute frosting has been around since at least the early 1900s. To make the unique texture this frosting is known for, egg whites and sugar are combined and beaten over simmering water, to dissolve the sugar and achieve a very light, fluffy texture.

1 1/2 cups (298g) granulated sugar
1/3 cup (74g) water, cold
2 large egg whites
2 teaspoons light corn syrup
1/4 teaspoon cream of tartar
pinch of salt
1 teaspoon King Arthur Pure Vanilla Extract

In the top of a double boiler, combine the sugar, water, egg whites, corn syrup, cream of tartar, and the pinch of salt.

Beat with an electric mixer at low speed for 30 seconds, then set the pan over (but not touching) simmering water. Continue beating at high speed for about 7 minutes, or until it's stiff and glossy. The mixture should measure at least 161°F (71.7°C) on a digital thermometer at the end of the 7 minutes.

Remove from the heat, add the vanilla, and beat for another 1 to 2 minutes.

Use right away, as the frosting will set quickly.

YIELD enough frosting to cover a 10" tube or 2- or 3-layer cake