Italian Buttercream

From the Syllabub blog at blogspot.com. Use this to fill layers of Chocolate Sour Cream Cake. Yum!

• Ready In: 20 mins
• Serves: 12
• Yields: 4 1/2 cups

Ingredients

• 1 1⁄4 cups sugar (about 8 3/4 ounces or 250 gm)
• 5  large  egg whites
• 1  pinch cream of tartar
• 1  lb unsalted butter, chilled
• 1  teaspoon pure vanilla extract
•  brandy or   liqueur, to taste

Directions

1. In a small saucepan over medium heat, bring sugar and 2/3 cup water to a boil. Continue boiling until syrup reaches 238F on a candy thermometer (soft-ball stage).
2. Meanwhile, place egg whites in the bowl of a standing mixer fitted with the whisk attachment, and beat on low speed until foamy. Add cream of tartar, and beat on medium-high speed until stiff but not dry; do not overbeat.
3. With mixer running, add syrup to whites in a thin stream, beating on high speed until no longer steaming, about 3 minutes. Add butter bit by bit, beating until spreadable, 3 to 5 minutes; beat in vanilla and brandy. If icing curdles, keep beating until smooth.