BASE: Combine crumbs, cinnamon and margarine; press onto bottom of 9-inch springform pan. Bake at 325, 10 minutes. BODY: Combine cream cheese and sugar, mixing at medium speed on electric mixer until well blended. Add eggs, one at a time, mixing well after each addition. Blend in sour cream and vanilla. Blend chocolate into 3 cups batter; blend liqueur and peel into remaining batter. Pour chocolate batter over crust. Bake at 350, 30 minutes. Reduce oven temperature to 325. Spoon remaining batter over chocolate layer; continue baking 30 minutes. Loosen cake from rim of pan; cool before removing rim of pan. Chill. Garnish with chocolate curls, if desired.