Make the bottom layer: Line an 8x8-inch square pan with parchment paper, leaving enough of an overhang to pull the bars out of the pan when ready to serve.
Add the butter, sugar, and cocoa powder to a double boiler. Heat over medium-low heat, whisking occasionally until the butter is completely melted and the ingredients are well combined.
Whisk in the beaten egg, whisking constantly as the egg cooks and the mixture slightly thickens.
Remove from the heat and then stir in the graham cracker crumbs, almonds, and coconut. Press the mixture into the prepared pan, forming a firm, even layer.
Make the middle layer: Using a hand mixer or stand mixer, cream together the butter, heavy cream, custard powder, and powdered sugar.
Beat until light and fluffy, then spread evenly over the bottom layer.
Make the top layer: Add the chocolate to a microwave-safe bowl. Heat at 50 percent power for 1 minute, then stir well to melt the chocolate. Continue to melt at 50 percent power for 20 second intervals, stirring after each time, until the chocolate is melted and smooth.
Once the chocolate is melted, stir in the butter. Then pour on top of the custard layer and use an offset spatula to spread in an even layer.
Refrigerate the bars for 15 minutes. Then remove from the refrigerator and use a sharp knife to score the chocolate into 16 slices, where they will be cut before serving. Scoring the chocolate will prevent the chocolate from cracking when you cut the chilled bars. Return the bars to the refrigerator and chill for at least 2 hours before serving.
Cut the bars into 16 squares before serving. Store leftovers in the refrigerator.
Bird’s Custard Powder is the traditional custard powder used in this recipe. It’s available in many grocery stores or online.
Refrigerate the bars while preparing the middle and top layers. It will make it easier to spread the layers evenly.