Ingredients

Makes about 4 1/2 cups

　　3 ounces fine-quality bittersweet chocolate (not unsweetened), chopped

　　1/2 cup water

　　1 cup sugar

　　4 large egg whites

　　1/4 teaspoon cream of tartar

　　4 sticks (2 cups) unsalted butter, cut into pieces and softened to a cool room temperature

　　2 1/2 tablespoons instant espresso powder dissolved in 1 tablespoon hot water

　　1/4 teaspoon salt

　　

Step 1

In a metal bowl set over a pan of barely simmering water melt chocolate, stirring until smooth and cool.

　　

Step 2

In a heavy 1 1/2-quart saucepan bring water with sugar to a boil, stirring until sugar is dissolved. Boil syrup, without stirring, until it registers 248°F. on a candy thermometer.

　　

Step 3

While syrup is boiling, in a bowl with an electric mixer beat whites with a pinch salt until foamy and beat in cream of tartar. Beat whites until they just hold stiff peaks and add hot syrup in a stream, beating. Beat mixture on medium speed until completely cool, 5 to 10 minutes. Beat in butter, 1 piece at a time, until mixture is thick and smooth. Beat in espresso mixture, chocolate and salt, beating until smooth. Buttercream may be made 2 days ahead and chilled in an airtight container. Bring buttercream to room temperature and beat before frosting cake.