Mousseline Buttercream

Makes 4 ½ cups
2 cups unsalted butter, room temperature
1 cup granulated sugar
¼ cup water
5 large egg whites (to equal 5 fluid ounces – use glass measuring cup)
½ + 1/8 teaspoon cream of tartar
3 fluid ounces of liqueur such as Grand Marnier
            Alternatives: 1.for chocolate use 5 ounces of melted and cooled bittersweet chocolate
                                  2. 6 ounces melted and cooled white chocolate
                                  3. Add up to ¾ cup fruit puree or curd.

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 ¾ cup sugar and ¼ cup water, stirring constantly, until the sugar dissolved and the mixture is bubbling. Stop stirring and reduce heat to low. (remove if using electric range).

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 ¼ cup sugar until stiff peaks form when the beater is raised slowly. Increase the heat and boil the syrup until a thermometer registers 248F to 250F (firm ball stage). Immediately transfer syrup to glass measure to stop the cooking.

If using as 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 cup. Lower the 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 liqueur or other flavor. Place in airtight container.  Re-beat lightly from time to time to maintain silky texture.

Store: 2 days room temperature, 10 days refrigerated, 8 months frozen.  Allow to come to room temperature before re-beating.