Combine crust mix from package, crushed crackers, 3 tablespoons cocoa powder, 1/3 cup toffee bits, 2 tablespoons sugar and melted margarine in bowl. Press over bottom and up side of 9-inch tart pan with removable bottom placed on baking sheet, or 9-inch springform pan. Place in freezer.
Melt bittersweet chocolate and 2/3 cup cream in saucepan until smooth. Remove from heat; stir in remaining toffee bits. Pour into crust; smooth top; freeze 15 minutes.
Beat cheesecake filling mix, remaining 1/4 cup cocoa and milk in bowl until thickness of sour cream, 3 minutes. Spread over firm chocolate layer. Refrigerate 1 hour.
Beat remaining cream, 2 tablespoons sugar in bowl until stiff peaks form. Spread on tart. Dust with cocoa. Makes 12 servings.