Mousseline Buttercream

2 cups of unsalted butter
1 cup sugar
1/4 liquid cup water
5 large egg whites
1/2 + 1/8 tsp cream of tar tar
3 oz liqueur - optional

Beat butter until smooth & creamy. Set aside in cool place.
Heat up 3/4 cup sugar and the 1/4 cup water. Let it boiled in 7 mins at low heat.
Beat egg whites until foamy, add cream of tar tar and beat until soft peaks form. Gradually beat in the remaining sugar until stiff peak form. Then slowly add the boiled syrup while beating at max speed. Make sure the syrup don't fall on the beaters. Beat in the butter at medium speed one spoon at a time. Increased the speed if the mixture looks slightly curdled. The mixture will turn soupy at first, continue beating and it will turn into beautiful butter cream.

Yield : 4 1/2 cups
Store : 2 days at room temperature, 10 days refrigerated, 8 months frozen. Allow to come to room temperature before rebeating.