Line an 8"x8" square pan with parchment paper, set aside.
Prepare a double boiler by bringing about two inches of water to a boil in a medium size pot. Combine the 1/2 cup butter, 5 tbsp cocoa powder, 1/2 cup sugar, and 1 tsp vanilla in a metal or glass heat proof bowl. Place the bowl on top of the pot of boiling water, make sure the water does not touch the bowl, then whisk the chocolate mixture until smooth and melted.
Add the egg, and whisk until combined. Cook for 2-3 minutes, until the chocolate mixture thickens to the texture of hot fudge sauce.
Remove from heat and add the graham cracker crumbs, coconut flakes, and chopped walnuts. Stir till combined, then press evenly into the prepared 8x8 pan. Chill in fridge while you prepare the middle layer.
For the Middle Layer:
Add 1/2 cup butter, heavy cream, and custard powder to the bowl of your stand mixer, fitted with the whisk attachment and beat until light and fluffy. (it will curdle at first, and be a bright yellow, just keep mixing until it gets fluffy and lighter in color.
Add the powdered sugar, in 1 cup increments, beating in between, until light and fluffy.
Spread on top of the crust layer. Place in fridge to chill while you prepare the top layer.
For the top layer:
Prepare the double boiler again, and place the chocolate squares, butter and heavy cream in a heat proof bowl (I just washed out the bowl I used for the crust layer and used that one again). Whisk until melted and smooth. Remove from heat and let cool on counter for 10-15 minutes.
Pour on top of the bars, and smooth out. Chill in fridge for at least 15-20 minutes to let the chocolate set.
Remove bars from pan and cut into small squares. Store in fridge.