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.)

Stir all ingredients in medium saucepan over medium heat until sauce boils and thickens, about 5 minutes. Cool slightly. (Can be made 2 days ahead. Cover and refrigerate. Rewarm before serving.)

Turn marquise out onto platter; peel off foil.

Cut into 3/4-inch-thick slices.

Place 1 slice on each plate. Spoon warm cherry sauce over and serve immediately.