In large metal bowl, combine yolks, 1/4 cup of the sugar, the cream and butter. Place over saucepan of simmering water. Cook, stirring constantly, until temperature reaches 160 degrees F on instant-read thermometer, about 4 to 7 minutes. Remove bowl from saucepan. Add chocolate; whisk until melted, replacing over saucepan if needed. If little pieces of cooked egg appear, strain through sieve into clean bowl.