2 cups unsalted butter (softened but cool - not runny and greasy)
5 large egg whites
1 cup sugar
1/2 + 1/8 teaspoon cream of tartar
1/4 cup water
3 fluid ounces liquor of choice or optional flavorings below
In a mixing bowl beat the butter until smooth and creamy and set aside in a cool place.
Have ready a heatproof glass measure near the range.
In a small heavy saucepan (preferably with a non stick lining) heat 3/4 cup sugar and the 1/4 cup water, stirring constantly, until the sugar dissolves and the mixture is bubbling. Stop stirring and reduce the heat to low. (If using an electric range remove from the heat.)
In another mixing bowl beat the egg whites until foamy, add the cream of tartar, and beat until soft peaks form when the beater is raised. Gradually beat in the remaining 1/4 cup sugar until stiff peaks form when the beater is raised slowly.
Increase the heat and boil the syrup until the thermometer registers 248°F to 250°F (the firm ball stage). Immediately transfer the syrup to the glass measure to stop the cooking.
If using a hand held mixer beat the syrup into the whites in a steady stream. Don't allow the syrup to fall on the beaters or they will spin it onto the sides of the bowl. If using a stand mixer, pour a small amount of syrup over the whites with the mixer off. Immediately beat at high speed for 5 seconds. Stop the mixer and add a larger amount of syrup. Beat at high speed for 5 seconds. Continue with the remaining syrup. For the last addition, use a rubber scraper to remove the syrup clinging to the glass measure. Lower speed to medium and continue beating up to 2 minutes or until cool. If not completely cool, continue beating on lowest speed.
Beat in the butter at medium speed 1 tablespoon at a time. At first the mixture will seem thinner but will thicken beautifully by the time all the butter is added. If at any time the mixture looks slightly curdled, increase the speed slightly and beat until smooth before continuing to add more butter.
Lower the speed slightly and drizzle in the liquor. Place in an airtight bowl. Rebeat lightly from time to time to maintain silky texture. Buttercream becomes spongy on standing. Makes 4 1/2 cups.
Will keep 2 days room temperature, 10 days refrigerated, 8 months frozen. Allow to come to room temperature completely before rebeating to restore texture or it will break down irretrievably.
May be flavored with 5 ounces of melted and cooled chocolate or 6 ounces of melted and cooled white chocolate, up to 3/4 cup lightly sweetened fruit puree (strawberry or raspberry) or orange, lemon or lime curd.
Source of recipe: Rose Levy Beranbaum "The Cake Bible"