5 lg. egg whites
1/2 tsp. cream of tartar
1 c. sugar
1 lb. soft unsalted butter
1 tbsp. vanilla extract

Place egg whites, sugar and cream of tartar in a metal bowl to your elastic mixer. Place bowl over hot water and mix with wire whip until sugar has dissolved.

Attach bowl to electric mixer and beat at high speed until mixture is cool and a thick meringue is formed. Continue to mix at high speed and slowly add soft butter, 1 tablespoon at a time. Continue to beat until mixture is fluffy (don't give up hope, it takes a long time). Add vanilla.