BASE: Combine crumbs and margarine; press onto bottom of 23 cm springform pan. Bake at 350, 10 minutes. BODY: In 1.5 quart heavy saucepan, melt caramels with milk over low heat, stirring frequently, until smooth. Pour over crust. Top with pecans. 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 in chocolate; pour over pecans. Bake at 350, 40 minutes. Loosen cake from rim of pan; cool before removing rim of pan. Chill. Garnish with whipped cream and additional finely chopped pecans, if desired.