In the top bowl of a double boiler off the heat, combine all the ingredients except the vanilla. Beat the mixture for about 1 minute to mix thoroughly. Rest the bowl over simmering water over medium heat and whisk constantly for 7 minutes, or until frosting shows stiff peaks and has tripled in volume. Lift the pan off the water and whisk vigorously until frosting has cooled and is thick and glossy.
NOTE:: Add ½ cup of smooth peanut butter to the finished frosting. This stays at its fluffiest if you first whisk together the peanut butter and about a cup of frosting. Then fold in the rest of the frosting gently with a rubber spatula.