Mousseline Buttercream - cooking recipe

Ingredients

2 cups unsalted butter (softened but cool, not runny and greasy)
5 large egg whites
1 cup sugar
1/2 teaspoon cream of tartar, plus
1/8 teaspoon cream of tartar
1/4 cup water
optional flavorings
5 6 ounces fruit puree or 6 ounces fruit curds

Preparation

In a mixing bowl beat the butter until smooth and creamy and set aside in a cool place.
Have all other ingredients at room temperature.
Have ready a heatproof pouring container ready for the sugar syrup.
In a small heavy saucepan 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 the bowl of a stand mixer unsing the whisk attachment, 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 heatproof container to stop the cooking.
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 heatproof container.
Lower speed to medium and continue beating up to 2 minutes or until the bottom of your mixing bowl feels cool to the touch.
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.
When butter is fully incorporated you may add any of the optional flavorings.
Place in an airtight bowl.
Rebeat lightly from time to time to maintain silky texture.
Buttercream becomes spongy on standing.
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.