For Crust: Place margarine in the microwave pan. Cook, uncovered, on HIGH for 45-60 seconds or until melted. Stir in crushed chocolate wafers until all is moistened. Press mixture firmly against bottom and sides of pan. Cook, uncovered, on HIGH for 11.5 minutes or until set, rotating pan a half-turn after 30 seconds.
For Filling: In medium mixing bowl combine cream cheese, sour cream, sugar, milk, and egg; beat with an electric mixer on medium speed just until smooth. Stir in 2 oz. melted chocolate and vanilla. Pour cream cheese mixture into crust. Cook, uncovered, on MEDIUM for 11-14 minutes or until a knife inserted 5 cm from edge comes out clean, giving dish a quarter-turn every 3 minutes. (Center will be slightly set, but not firm.) Cool slightly, then refrigerate at least 4 hours or until set.
For Topping: (optional) In a 1.75 quart casserole cook remaining 1 oz. chocolate and shortening, uncovered, on HIGH for 2-3 minutes or until melted; stir until smooth. Drizzle chocolate mixture on top of cheesecake.
Per serving: 405 cal, 30 g fat, 86 mg chol