BASE: Combine crumbs, pecans, granulated sugar and margarine; press onto bottom of 23 cm springform pan. Bake at 325, 10 minutes. BODY: Combine cream cheese and brown sugar, mixing at medium speed until well blended. Add eggs, one at a time, mixing well after each addition. Blend in chocolate and liqueur; pour over crust. Bake at 325, 35 minutes.
TOPPING: Increase oven temperature to 425. Combine sour cream and granulated sugar; carefully spread over cheesecake. Bake at 425, 10 minutes. Loosen cake from rim of pan; cool before removing rim of pan. Chill. Garnish with chocolate leaves, if desired.