1. Prepare the Bottom Layer:

• In a medium bowl, combine the melted butter, granulated sugar, and cocoa powder. Stir until smooth.
• Add the beaten egg and mix well. Pour in the graham cracker crumbs, shredded coconut, and walnuts (if using). Stir until everything is well combined.
• Press the mixture firmly into the bottom of an ungreased 9×9-inch (or similar size) baking pan. Use the back of a spoon to press it down evenly.

2. Make the Custard Layer:

• In a medium bowl, beat the powdered sugar, custard powder, softened butter, and milk until smooth and creamy.
• Spread the custard mixture evenly over the graham cracker base, smoothing it out with a spatula. Chill in the refrigerator for about 30 minutes to set.

3. Prepare the Top Layer:

• Melt the semi-sweet chocolate and butter in a microwave-safe bowl in 30-second intervals, stirring between each, until completely smooth.
• Pour the melted chocolate over the custard layer and spread it evenly with a spatula.
• Refrigerate the bars for at least 1 hour, or until the chocolate layer is fully set.