Base
Filling
Topping
In a bowl, stir together graham crumbs, coconut, walnuts, cocoa powder, and sugar. Drizzle with butter and egg, stirring until combined.
Press into parchment paper–lined 9-inch square cake pan. Bake in 350°F oven until firm, about 10 minutes. Let cool in pan on rack.
Filling: In a bowl, beat together butter, custard powder, and vanilla. Beat in icing sugar alternately with milk, making 3 additions of sugar and 2 of milk and adding up to 1 tsp more milk if too thick to spread. Spread over cooled base. Refrigerate until firm, about 1 hour.
Topping: In a heatproof bowl over a saucepan of hot (not boiling) water, melt chocolate with butter; spread over filling. Refrigerate until chocolate is almost set, about 30 minutes.
With the tip of a knife, score into bars; refrigerate until the chocolate is completely set, about 30 minutes. Cut into bars.
