Place all ingredients except vanilla in top of double boiler.
Beat 1 minute with electric mixer or rotary beater.
Set over boiling water, and beat constantly with rotary or electric beater until mixture forms peaks, about 7 minutes.
Remove from heat, and add vanilla.
Beat until mixture is of spreading consistency.
This amount will frost top and sides of 2 9-inch layers or 1 10-inch tube cake.
For smaller amount, halve all ingredients, and cook about 4 minutes.
Frost top of average loaf cake, top and sides of 8-inch tube cake or 8-10 cupcakes.