To make the crust, melt the butter in a heavy saucepan over low heat, and whisk in the sugar, cocoa, and eggs. Stir in the coffee liqueur, and whisk constantly until the mixture thickens, about 2 minutes. Mix the graham cracker crumbs with coconut and walnuts in a bowl, and stir into the cocoa mixture. Grease a 9x13-inch square dish or line with greased waxed paper, and pack the crumb mixture into the bottom of the dish. Refrigerate the crust while making the filling.
For the custard filling: place butter, confectioners' sugar, custard powder, and Irish cream liqueur into a mixing bowl, and beat with an electric mixer until the mixture is smooth and creamy. Spread the filling over the crust, and refrigerate the dish until the filling has set, at least 1 hour.
For the topping: Melt the chocolate chips and 2 tablespoons of butter in the top of a double boiler over hot water, stirring until the mixture is smooth and well blended. Stir in the orange liqueur, and drizzle the chocolate topping over the custard filling to cover completely. Return the dish to the refrigerator, and chill until the topping has hardened, at least 1 hour. To cut, run a sharp knife under hot water and cut the bars with the heated knife, reheating the knife as necessary.