Crust: Mix 1 cup butter, white sugar, cocoa powder, and eggs in a heavy saucepan or double boiler. Once eggs have been incorporated into mixture, add Kahlua(R) and cook over low heat until mixture is of a custard like consistency.
Combine graham wafer crumbs, coconut and chopped walnuts together and then add to the melted mixture. Mix well and pack into a buttered 13 x 9 inch cake pan. (You can also line with greased wax paper for easy removal if you like, only remove the wax paper before cutting).
Filling: Cream 1/2 cup butter, confectioner's sugar, custard powder, and Bailey's Irish Cream(R). Beat until creamy and spread over the base crust. Refrigerate until hardened.
Topping: Melt semi-sweet chocolate chips and butter in a double boiler. Quickly stir in Grand Mariner(R). Drizzle over the custard filling until completely covered. Refrigerate until hard.
Cut into 2 x 1 inch bars with knife. Run knife under hot water periodically to make cutting easier.