Base: In bottom of double boiler, bring about 2” of water to the simmer point (around 200°F). Maintain the water at this simmer point over medium-low heat. In the top of the double boiler, melt the butter and then add the sugar and cocoa powder.  Stir to combine ingredients.  Whisk in the egg.  Stir until mixture thickens.  Remove saucepan from heat and stir in the graham cracker crumbs, coconut, and nuts. Press mixture into prepared pan. Cool in refrigerator for 10-15 minutes.

Filling: In stand mixer fitted with paddle attachment, cream the butter and whipping cream together. Add the vanilla. Blend in the custard powder followed by the icing sugar, beating until mixture is smooth. If mixture appears too stiff, add 1-2 teaspoons whipping cream and beat mixture well to combine (note, however, that mixture should not be soupy). Spread evenly over cooled base. Refrigerate 1-2 hours.

Topping: In small saucepan, over low heat, melt the chocolate and butter together, stirring to mix. Spread evenly over chilled filling. Refrigerate to set chocolate, approximately 30 minutes. Use a sharp, flat blade knife to cut into squares.