1. Preheat oven to 350 degrees F.
2. Combine the graham cracker crumbs with the coconut, walnuts, cocoa powder, and sugar. Add in the butter and egg and stir to combine.
3. Press this mixture into a 9-inch square baking pan that you have lined with parchment paper. Bake for about 10 minutes. You want this crust to be firm. Place the pan on a wire rack to cool

FILLING

1. Place the butter, custard powder, and vanilla in a mixing bowl and beat. Continue beating while adding in the icing sugar, alternating with milk. You should have a smooth mixture. If needed, you can add more milk if needed (up to 1 tsp).
2. Spread the filling over the cooled crust. Place in the refrigerator to firm up. This will take about an hour.

TOPPING

1. In a double boiler, or using a heatproof bowl over a saucepan of hot water, melt the chocolate with the butter. Important: the water needs to be hot but it should not be boiling.
2. Once melted, spread it over the chilled filling. Place back in the refrigerator for about half an hour. You want the topping to be nearly set.
3. Using the tip of a knife (don't cut right into it), score into bars. Once more, place back into the fridge until fully set - about an hour.
4. You can keep these in the refrigerator for about 4 days or in the freezer for about 2 weeks.