Cool in pan 10 minutes. Carefuly loosen sides with spatula; remove from pans; cool completely on wire racks. FROSTING: In medium saucepan, combine chocolate pieces, cream and butter; stir over medium heat until smooth. Remove from heat. With wire whisk, blend in powdered sugar. In bowl set over ice, beat until frosting holds it shape. FILLING: Whip cream with powdered sugar and vanilla: refrigerate. ASSEMBLE: On plate, place a layer top side down; spread with half of the filling. Place second layer top side down; spread with remaining filling. Place third layer top side up. With spatula, frost sides first, covering whipped cream filling; frost remaining cake, swirling decoratively. Refrigerate at least 1 hour before serving. To cut, use a thin edged knife and slice with a sawing motion. ----