Line an 8x8 inch pan with foil or parchment paper.
In a medium pot on low heat, warm butter, sugar, and cocoa and stir until smooth, about 3-5 minutes. Stir in egg and vanilla until well combined.
Remove from heat and stir in graham crumbs, coconut, and walnuts until well combined. Spread in prepared pan and refrigerate to set while preparing the middle layer.
Middle Layer
Using a stand mixer and bowl (or hand mixer and medium bowl), cream butter for about 1 minute. Add powdered sugar, custard powder, and milk and beat to combine, about 2 minutes.
Using an offset spatula, spread custard mixture over the bottom layer, smoothing the top, and return to the refrigerator to set while preparing the top layer.
Top Layer
To a small bowl, add chocolate and butter. Microwave on 50% heat for 15 seconds and stir. Microwave in 10-second increments until chocolate is melted and smooth. Carefully spread hot mixture thinly over the custard layer*.
Refrigerate until chocolate is firm. Cut into small squares and serve**.
Notes
*To spread easily, working quickly, pour the chocolate on the Middle Layer and tip the pan in all directions to spread the chocolate.**For the cleanest cuts, run a long (serrated) knife under hot water. Dry with a towel, and cut pressing straight down on the knife. Clean and run under water for each cut.
Double the top layer to make it easier to spread and if you like more chocolate.