- Place the egg whites in a large bowl. Set aside.
- In a small saucepan, bring the sugar, water, and corn syrup to a boil. Simmer until a candy thermometer reads 118 °C (244 °F). Remove from the heat for a moment to beat the egg whites.
- Beat the egg whites and vanilla with an electric mixer at medium speed until soft peaks form. Drizzle the hot syrup over the eggs, avoiding the beaters of the mixer. Beat constantly until the meringue is lukewarm and stiff peaks form.
- Add the butter, one or two cubes at a time, beating until very smooth. Scrape the bowl several times. Set aside.
The lightness, both in the color and the texture of this buttercream, is because the recipe is made with only egg whites, instead of whole eggs, like in a traditional buttercream.