- Preheat the oven to 350°F. Lightly grease an 8-inch square pan and line it with parchment paper so that it comes up the sides.
- For the base, stir the melted butter and sugar. Sift the cocoa powder into the butter mixture and blend well. Whisk together egg and vanilla extract and add in. Blend in chocolate cookie and graham crumbs, coconut and walnut pieces (if using) until evenly incorporated. Press th into the prepared pan and bake for 10 minutes. Allow to cool for 10 minutes then chill for 20 minutes.
- For the filling, beat the butter by hand until smooth and then beat in the custard powder. Stir in the milk, vanilla, orange extract and a little orange food colouring and add sifted icing sugar a cup at a time, combining well. Spread th over base and chill while preparing topping.
- For the topping, melt chocolate and butter over a pot of gently simmering water while stirring gently. Pour th over the filling and spread to cover evenly. Chill for 30 minutes. To serve, slice with a hot, dry knife into squares.