Line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. In a medium saucepan over low heat, combine the melted butter, granulated sugar, cocoa powder, and beaten egg. Stir constantly for 2-3 minutes until the mixture thickens slightly.
Remove from heat and stir in the gluten free graham cracker crumbs, shredded coconut, and chopped walnuts until everything is evenly combined. Press the mixture firmly and evenly into the prepared pan using the bottom of a measuring cup. Refrigerate for 20 minutes.
Using an electric mixer, beat the softened butter, heavy cream, custard powder, and powdered sugar together until light, fluffy, and smooth, about 2-3 minutes.
Spread the filling evenly over the chilled base using an offset spatula. Return the pan to the refrigerator for another 20 minutes until the filling is firm.
Place the semi-sweet chocolate chips and 2 tablespoons of butter in a microwave-safe bowl. Microwave in 30-second intervals, stirring between each, until completely smooth and melted.
Pour the chocolate over the custard layer and gently tilt the pan to spread it evenly. Refrigerate for at least 1 hour until the chocolate is fully set.
Use the parchment overhang to lift the bars out of the pan. Let them sit at room temperature for 5 minutes before slicing into squares with a sharp, smooth edge. Store in the refrigerator.