Combine chocolate and powdered sugar in a medium saucepan; mix well. Gradually stir in 1 cup whipping cream; bring to a boil, stirring frequently. Remove from heat, and set aside.
Soften gelatin in cold water. Add boiling water; stir until gelatin dissolves. Add dissolved gelatin, sugar, and vanilla to chocolate mixture; mix well. Transfer to a large mixing bowl. Chill until consistency of unbeaten egg whites.
Fold in whipped cream, and pour mixture into a lightly greased 12-cup mold. Freeze overnight.
Unmold onto a serving platter. Slice and serve on individual serving plates.