Mix vanilla wafer crumbs, margarine and 2 tablespoons Splenda in bottom of 9-inch springform pan. Reserve 1 tablespoon crumb mixture. Pat remaining mixture evenly on bottom and 1/2 inch up side of pan. Bake in preheated 350 degree F oven until crust is lightly browned (approximately 8 minutes). Cool on wire rack. Reduce oven temperature to 300 degrees F.
In a large bowl, beat cream cheese and 3/4 cup Splenda until fluffy; beat in eggs, egg whites and cornstarch. Stir in sour cream and vanilla until well mixed. Remove 1/2 cup cheesecake mixture. Pour remaining mixture into crust in pan.
Add melted chocolate to 1/2 cup reserved cheesecake batter;mix well. Place dollops of chocolate mixture on top of cheese cake. Using butter knife or spatula, gently swirl chocolate mixture into cheesecake.
Wrap bottom of springform pan with aluminum foil and place in roasting pan on middle oven rack; add 1 inch hot water to pan. Bake in preheated 300 degree F oven just until set in center, (45 to 60 minutes). Remove cheesecake from roasting pan, sprinkle with reserved crumbs and return to oven. Turn oven off and let cheesecake cool in oven with door ajar for 3 hours. Refrigerate 8 hours or overnight.
Remove side of springform pan; place cheesecake on serving plate.