Grease an 8-inch (20cm) square baking tin and line with parchment paper. Set aside.
Base
Place the butter, sugar, and egg in a heat-safe glass bowl. Place the bowl over a small saucepan of simmering water (don’t let the bottom of the bowl touch the water). Heat, stirring constantly, for about five minutes. It should thicken to the consistency of custard.
Remove the bowl from the heat and add the vanilla, cocoa, graham wafer crumbs, coconut, and walnuts. Stir well to combine.
Scoop the mixture into the prepared pan. Using a spatula or spoon, press it evenly into the pan. Place this into the refrigerator to firm while you prepare the filling.
Filling
To make the custard filling, add the butter, milk, icing sugar, and custard powder to a large bowl. Mix on high speed with electric beaters until light and fluffy.
Spread the filling over the chilled base in an even layer. Place in the refrigerator for about 20 minutes to firm again.
Topping and Slicing
Add the chopped chocolate and butter to a bowl. Melt, using a microwave, or with a double boiler (as outlined for the base).
Stir to combine, then pour over the chilled filling, spreading as needed into an even layer.
Place the tin back into the refrigerator to set completely, another 15 minutes.
Once the chocolate has set, slice the bars. Use a hot, dry knife to cut 24 small slices.
Keep the bars in the refrigerator for up to 10 days, or freeze up to one month. Thaw in the refrigerator if possible as the filling softens at room temperature.
Notes
Using a hot knife is necessary to get clean slices. Otherwise the chocolate will crack and the soft custard filling will push out, losing the straight edges. Run your knife under hot water and dry well before slicing.