Set a heatproof bowl over a pot of gently simmering water. Add the melted butter, cocoa powder, and sugar, and stir until smooth, about 3 minutes.

Slowly whisk in the beaten egg, stirring constantly, and cook for 2 to 3 minutes until the mixture thickens and coats the back of a spoon. Stir the whole time here, or the egg scrambles instead of thickening smoothly.

Remove from heat. Stir in the graham cracker crumbs, shredded coconut, and chopped walnuts until fully combined.

Press the mixture firmly and evenly into a parchment-lined 23 x 23 cm / 9 x 9-inch baking pan, using a flat-bottomed glass to pack it tight. Chill for 20 minutes.

Beat the softened butter in a bowl with a hand mixer until pale, about 1 minute.

Add the custard powder, vanilla, and milk, and beat until combined.

Gradually add the powdered sugar, about a third at a time, beating until light and spreadable. Add an extra teaspoon of milk if it looks too stiff.

Spread the custard mixture evenly over the chilled base with an offset spatula. Chill for 30 minutes until firm to the touch.

Melt the chocolate and butter together in the microwave in 30-second bursts, or over a double boiler, stirring until smooth.

Pour the melted chocolate over the chilled custard layer and tilt the pan so it spreads evenly to the edges.

Chill for 10 minutes until the surface turns matte, then score into 20 bars (5x4 grid) with a sharp knife before the chocolate fully hardens.

Chill for at least 1 hour more, then cut all the way through along the scored lines, wiping the knife clean between cuts. Serve cold.