Bittersweet Chocolate Marquise with Cherry Sauce

Butter 8 1/2x5 1/2x3-inch glass loaf dish. Line dish smoothly with foil. Stir chocolate in top of double boiler over barely simmering water until smooth. Turn off heat. Using electric mixer, beat 1/2 cup butter in large bowl until fluffy. Beat in 1/4 cup sugar, then cocoa powder. Whisk yolks, 1/4 cup water, 1/4 cup butter, and 1/4 cup sugar in metal bowl. Set over saucepan of simmering water (do not let bowl touch water). Whisk constantly until candy thermometer registers 160 °F, about 6 minutes. Remove from over water. Using electric mixer, beat yolk mixture until thick and cool, about 5 minutes. Beat into cocoa mixture. Fold in warm chocolate and vanilla. Beat cream in another bowl until soft peaks form. Fold into chocolate mixture; spread in prepared dish. Cover and chill until firm, at least 4 hours. (Can be made 4 days ahead. Keep chilled.)