sugar water egg whites cream of tartar unsalted butter vanilla
Bring sugar and water to a boil and allow it to reach 240 degrees F.
Beat egg whites on low to medium speed until foamy.
Add cream of tartar and whip until stiff peaks form.
Add the sugar syrup in a steady stream and beat until bowl is cool to touch.
Add butter a tablespoon at a time, beating between additions.
Beat in vanilla until fully incorporated.