WHITE CHOCOLATE MOUSSELINE BUTTERCREAM (Rose Levy Beranbaum's recipe)

adapted from Rose Levy Beranbaum's recipe. delicious!

2 cups/1 lb/454 grams unsalted butter, softened but cool (65degreesF)
1 cup/7 oz/200 grams sugar
1/4 liquid cup/2 oz/60 grams water
5 large/5.25 oz/150 grams egg whites, room temp.
1/2 + 1/8 tsp. cream of tartar
1 tsp. pure almond extract
6 oz melted and cooled white chocolate, pref. Tobler Narcisse

Makes 4.5 cups/1 lb 14 oz/858 grams (enough to fill and frost two 9-inch by 1 1/2-inch layer or three 9-inch by 1-inch layers)

• 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 heat 3/4 cup sugar and 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 egg whites until foamy, add 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 a thermometer registers 248degreesF to 250degreesF (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 almond extract. Beat in 6 ounces melted and cooled white chocolate. Place in an airtight bowl. Rebeat lightly from time to time to maintain silky texture. Buttercream becomes spongy on standing.