BASE: Combine crumbs, sugar and margarine; press onto bottom of 9inch springform pan. Bake at 350, 10 minutes. BODY: Combine cream cheese, sugar and vanilla, mixing at medium speed on electric mixer until well blended. Add eggs, one at a time, mixing well after each addition. Blend chocolate into 1 cup batter. Spoon plain and chocolate batters alternately over crust; cut through batters with knife several times for marble effect. Bake at 450, 10 minutes. Reduce oven temperature to 250; continue baking 30 minutes. Loosen cake from rim of pan; cool before removing rim of pan. Chill.