Place 1/2 cup butter, 1/4 cup sugar, and 3 Tbsp cocoa powder into a large, heatproof bowl over a pot with simmering water. Whisk until combined.
Slowly pour in egg while whisking vigorously. Add vanilla. Continue whisking over simmering water until mixture has thickened and resembles a pudding.
Remove mixture from heat and stir in graham cracker crumbs and coconut.
Spread mixture evenly into the bottom of your 9x9 pan. Press down firmly to pack in. Chill in fridge while making custard.
Cream together butter and powdered sugar, add in custard powder and milk and beat until smooth.**
Spread evenly over base layer and return to fridge. Chill for 30mins.
Place chopped chocolate and butter in to a microwave safe bowl. Microwave for 30 seconds, stir. Continue microwaving in 10 second intervals, stirring in between, until chocolate has melted.
Spread evenly over custard layer. Chill in fridge until set (about 1 hour).
Cut with a hot serrated knife.
* The original recipe calls for half this amount of chocolate & butter. I doubled it as I prefer a thicker chocolate layer, but this will make them harder to cut.** This can be done by hand but I used my stand mixer.