- 2 cups sugar
- 6 large egg whites
- Pinch of salt
- 1/2 teaspoon cream of tartar
- 1 1/4 pounds (5 sticks) unsalted butter, room temperature
- 1/4 cup Grand Marnier or 1 tablespoon pure vanilla extract
In a small, heavy-bottomed saucepan, mix together sugar and 2/3 cup water. Bring to a boil and do not stir. Cover and boil until no sugar crystals remain, 2 to 3 minutes. Uncover and continue boiling until syrup reaches 240 degrees on a candy thermometer (soft-ball stage).
Meanwhile, in the bowl of an electric mixer fitted with the whisk attachment, beat together egg whites and cream of tartar until stiff peaks form. When sugar mixture has reached 240 degrees, add 1/4 cup to bowl of mixer, drop by drop. Slowly add remaining sugar mixture and continue beating until cool, 7 to 10 minutes.
In the clean bowl of an electric mixer fitted with the paddle attachment, cream butter until fluffy. Add creamed butter to cooled egg white mixture, 3 tablespoons at a time, whisking well after each addition. Frosting should become thick and creamy. If egg mixture is too warm, frosting may remain thin; set bowl over ice and stir until thickened. Add Grand Marnier and mix until well combined. Use frosting immediately.