Grease and line a loaf pan with parchment paper. Set aside.
Place the graham cracker crumbs, walnuts and coconut in a large bowl and stir to combine. Set aside.
In a small saucepan, melt the butter for layer 1. Whisk in the sugar and cocoa and stir to combine. Remove the mixture from the heat and slowly stream in the egg, whisking constantly.
Add the cocoa mixture to the coconut mixture and stir until a crumbly dough forms. Press the mixture into the loaf tin to form a thin, even layer. Set aside.
For layer 2, place the softened butter into a stand mixer fitted with a whisk attachment. Add the cream and custard powder and whisk to combine.
Set the mixer to low and start adding the icing sugar a 1/4 cup at a time. Once the sugar is integrated, whip the mixture on high speed until well-combined.
Spread a quarter of the mixture over top of your walnut, graham cracker layer and set aside. Transfer the remaining mixture to a bowl and refrigerate until ready to use.
For layer 3, melt the chocolate and the butter in a small saucepan over low heat. Remove the mixture from the heat and let cool slightly.
Pour the mixture over the second layer and transfer the finished Nanaimo bars to the fridge. Let set for 1 hour before slicing. Once sliced, return the bars to the fridge for another hour.