PREHEAT oven to 325° F. Tightly wrap outside bottom and side of 9-inch springform pan with 2 pieces of foil to prevent leakage. Lightly grease inside of pan.
COMBINE crumbs and butter in small bowl. Press onto bottom of prepared pan. Bake for 10 minutes. Cool on wire rack.
BEAT cream cheese, sugar, flour and vanilla extract in large mixer bowl until well blended. Beat in melted chocolate and liqueur, if desired. Add eggs, one at a time, beating well after each addition. Pour onto crust. Place pan in large roasting pan; fill roasting pan with hot water to 1-inch depth.
BAKE for 60 to 70 minutes or until edge is set but center still moves slightly. Remove cheesecake from water bath to wire rack. Run knife around edge of cheesecake. Cool completely. Refrigerate for 4 hours or overnight.
TO GARNISH AND SERVE:
LINE baking sheet with wax paper.
PLACE morsels in heavy-duty plastic bag. Microwave on HIGH (100%) power for 30 to 45 seconds; knead. Microwave at 10- to 15-second intervals, kneading until smooth. Cut tiny corner from bag. Drizzle 12 to 16 circular designs about 2 inches high and wide onto prepared baking sheet. Refrigerate for 5 to 10 minutes or until chocolate is firm.
REMOVE side of pan. Place cheesecake on serving platter. Spoon (or use pastry bag fitted with fluted tip) dollops of whipped cream around edge of cheesecake. Remove chocolate drizzle designs from refrigerator. With tip of knife, gently remove designs from wax paper and insert, standing up, into dollops. Serve immediately.