Line a 9 x 9-inch square pan with parchment paper, leaving enough overhang on two sides to lift out the finished bars.

Melt the butter in a saucepan over low heat. Whisk in the sugar and cocoa powder, then add the lightly beaten egg and cook for 3–5 minutes, stirring constantly, until slightly thickened without allowing the mixture to boil.

Remove the saucepan from the heat and stir in the vanilla, graham-cracker crumbs, coconut, and chopped nuts. Press the mixture firmly and evenly into the lined pan, then chill for 20–30 minutes.

Beat the softened butter until smooth for the custard layer. Add the cream or milk, custard powder, sifted icing sugar, and vanilla, beat until light and creamy, then blend in the melted white chocolate.

Spread the white custard mixture over the chilled base, taking it all the way to the corners. Refrigerate for at least 30 minutes, or until the center feels firm.

Melt the topping chocolate and butter together in short microwave intervals, stirring between each interval, or melt them in a heatproof bowl over barely simmering water. Spread the mixture over the firm custard layer.

Chill the bars for 1–2 hours, until the topping is completely set. Let them stand at room temperature for 10–15 minutes before cutting, then use a sharp knife warmed under hot water and wiped dry between cuts.