Line an 8in / 20cm square pan. Stir 125g of softened butter, 50g of superfine sugar, and 5 tablespoons of cocoa powder in a heatproof bowl set over a pan of simmering water until melted.
Whisk in 1 beaten egg and keep whisking until the mixture thickens. Off the heat, stir in 200g of crushed digestive biscuits and 100g of desiccated coconut. Press into the pan and chill while you make the custard layer.
Beat 100g of butter, 4 tablespoons of heavy cream, and 3 tablespoons of custard powder until fluffy. Gradually beat in 250g of icing sugar, spread over the base, and chill until firm.
Melt 150g of dark chocolate with 50g of butter in the microwave and spread over the custard layer. Refrigerate for about 2 hours, until set, then lift out and cut into squares.
