Prepare the Bottom Layer:

In a medium saucepan, combine 1/2 cup butter, granulated sugar, and cocoa powder. Heat gently until melted and smooth.
Slowly drizzle in the beaten egg, stirring constantly until the mixture thickens.
Remove from heat and stir in graham cracker crumbs, coconut, and nuts (if using).
Press the mixture firmly into an ungreased 8×8-inch square baking pan. Chill in the refrigerator while preparing the next layer.
Make the Middle Layer:

In a bowl, beat together 1/2 cup butter, custard powder, powdered sugar, and milk until light and fluffy.
Spread this custard mixture over the bottom layer in the pan. Return to the refrigerator to chill until firm, about 1 hour.
Prepare the Top Layer:

In a small saucepan, melt the chocolate and 2 tablespoons butter together over low heat, stirring until smooth.
Let the chocolate cool slightly, then pour over the custard layer, spreading evenly.
Chill the bars in the refrigerator until the chocolate is set.
Cut and Serve:

Once set, cut into squares or bars.
It’s easier to cut the bars if you let them sit at room temperature for a few minutes.
Storage:

Nanaimo Bars can be stored in an airtight container in the refrigerator for up to a week or frozen for longer storage