Line a 9x9 pan with parchment paper, leaving some parchment paper hanging over the edge for handles.
In a large pot over medium heat, melt the butter, sugar, and cocoa together until it's smooth.
Quickly beat in the egg, making sure there are no lumps.
Mix in the graham crumbs, coconut, and walnuts. Firmly press into the bottom of the prepared 9x9 pan. Chill in the refrigerator while you get the middle layer ready.
Get your middle ingredients together.
Cream the icing, butter, Bailey's, and custard powder together with a hand mixer, then spread onto the bottom layer.
Melt the cup of chocolate chips and 2 tbsp of butter together in a microwave-safe glass measuring cup, heating on high power in 20-second increments, making sure to stir after heating each time.
Once the chocolate is smooth, spread the chocolate on top of the middle layer evenly, then cool in the fridge for a few hours. The middle layer will set and the top layer will harden.
Remove the slab of bars from the pan once chilled by lifting the parchment paper. Slice the bars on the paper and serve.
Keep the bars refrigerated in a closed container for up to 5 days, or freeze for up to 6 months.