3/4 cup (1 1/2 sticks) unsalted butter, cut into pieces, room temperature
1/2 teaspoon vanilla extract
Directions
Mix sugar, yolks, and syrup in small bowl. Bring cream to simmer in heavy 3-quart saucepan. Add both chocolates and stir over low heat until smooth. Remove from heat. Stir in butter, then sugar mixture. Cook over low heat until mixture registers 165°F to 170°F on candy thermometer, stirring constantly, about 10 minutes. Immediately pour mixture into bowl; do not scrape sides of pan. Add vanilla.
Using electric mixer, beat frosting until cool. Let frosting stand at room temperature until thick enough to spread, stirring occasionally, 1 1/2 hours.
