Process first 3 ingredients in a food processor until walnuts are ground, stopping to scrape down sides. Add melted butter, and process until blended. Press mixture firmly onto bottom and up sides of a 9-inch pieplate.
Bake at 350° for 12 to 15 minutes or until lightly browned. Cool.
Beat cream cheese and butter at medium speed with an electric mixer until smooth; add 1 2/3 cups powdered sugar, beating until blended. Add chocolate and liqueur, beating until blended. Spoon into crust, and chill 8 hours. Store in refrigerator up to 4 days, if desired.
Beat whipping cream at medium speed with electric mixer until foamy; add 1 tablespoon powdered sugar, beating until stiff peaks form. Spread on top of pie just before serving.