Preheat oven to 325º F.

Mix cookie crumbs and butter together; press onto bottom of a 9-inch springform pan. Bake for 10 minutes. Remove from oven and cool.

Beat cream cheese, sugar, flour, and vanilla with electric mixer until well blended. Add chocolate, liqueur (if desired) and eggs. Mix just until blended. Pour over crust.

Bake for one hour or until center is almost set. Run knife around rim of pan to loosen cake. Cool before removing from pan. Refrigerate 4 hours or overnight. Yield: 12 servings.