BASE: Combine crumbs and margarine; press onto bottom of 9-inch springform pan. Bake at 350, 10 minutes. BODY: Combine cream cheese, sugar and flour, 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. Stir in chocolate pieces; pour over crust. Bake at 325, 55 minutes. Loosen cake from rim of pan; cool before removing rim of pan. Chill. Garnish with whipped cream and fresh mint, if desired.