Use this Swiss meringue to create rosettes atop the Baked Stuffed Apples recipe from "Martha's Entertaining."
In a metal mixing bowl, whisk egg whites and cream of tartar to break up egg whites. Set bowl over (not in) a pan of simmering water (do let bottom of bowl touch water, or eggs may begin to set). Whisk until whites are foamy, then gradually whisk in sugar, and continue whisking until mixture is completely smooth (test by rubbing some between your fingertips).
Attach bowl to standing mixer and whisk on low speed 5 minutes. Increase speed to high, and whisk until meringue is stiff and glossy, about 5 minutes more. Mix in vanilla and use immediately.