It's from my favorite book - Toba Garrett's "The Well Decorated Cake". It makes alot, so for a 9" round double-layer with simple decorations I make half the recipe. Also, if the weather is really warm, I substitute about half a pound of shortening for half a pound of the butter (for the whole recipe).
10 egg whites (~1 1/2 cups)
3 cups granulated sugar
3 lbs. unsalted butter, at room temp, cut into 2" chunks
1 tbsp. vanilla
1/4-1/3 cup Frangelico
1 recipe white chocolate ganache (recipe follows)
Whisk egg whites and sugar together in top of a double boiler. Heat until it reaches 140 degrees on a candy thermometer.
Transfer to a mixing bowl and whip on med/high until doubled in volume and stiff peaks form. It shouldn't be dry looking - it should be very glossy.
Using the paddle attachment, add half the butter and pulse the mixer a couple of times until the butter is completely covered with meringue. Add the rest of the butter and pulse a few more times. Slowly increase the mixer's speed, starting at the lowest speed, every 10 seconds until you reach the med/high speed. Continue beating until it's light and fluffy - usually 3-5 minutes for me. Reduce the speed to low and beat in the Frangelico. Still on low, beat in the ganache about 1/2 cup at a time.
white chocolate ganache:
3/4 cups cup heavy cream
1/2 lb. white chocolate, chopped
Bring cream to a boil. Turn off heat and add chocolate. Let rest until chocolate is melted. Stir well, then refrigerate until firm.
Toba says you can freeze it for up to three months, but I've never tried that. I love this recipe because it pipes well and tastes awesome! It is very rich, but I'd rather use this type of buttercream over the butter/10x sugar variety. I hope you like it!