Butter an 8 by 8-inch baking dish. Line with parchment paper, leaving an overhang on two sides, and butter the parchment. Set aside.

For the Base Layer: In a large heat-proof bowl set over a saucepan of simmering water (a double boiler), melt the 0.5 cup unsalted butter. Remove from heat.

Add granulated sugar, cocoa powder, and salt to the melted butter. Whisk until well combined. While whisking vigorously, slowly pour in the lightly beaten egg in a thin stream.

Return the mixture to the heat and cook, stirring constantly, until the mixture has just thickened (about 3-5 minutes). Be careful not to scramble the egg.

Remove from heat and stir in the graham cracker crumbs, finely chopped almonds, and sweetened shredded coconut. Toss until all ingredients are evenly coated.

Press the mixture firmly and evenly into the prepared baking dish. Chill in the refrigerator for at least 15 minutes while you prepare the filling, ensuring the base is cool before adding the next layer.

For the Custard Filling: In a mixing bowl, combine the 0.5 cup softened unsalted butter, powdered sugar, Bird's custard powder, and 2 tablespoons heavy cream. Using an electric mixer with a paddle attachment, beat on medium speed until the mixture is very pale and fluffy. Add an additional 1 tablespoon of cream if needed to achieve a smooth, spreadable consistency.

Spread the custard filling evenly over the chilled base layer. Chill in the freezer for about 15-20 minutes, or in the refrigerator for 30-45 minutes, to firm up before adding the chocolate topping.

For the Chocolate Topping: In a heat-proof bowl set over simmering water (or in a microwave-safe bowl, heating in 20-second intervals and stirring between each), melt the chopped semi-sweet chocolate and 2 tablespoons unsalted butter until smooth.

Pour the melted chocolate mixture over the chilled custard layer and spread quickly and evenly. Ensure the topping covers the entire surface.

Chill the Nanaimo bars in the refrigerator for at least 1-2 hours, or until the chocolate topping is completely set. For easier cutting, let the bars sit at room temperature for 10-15 minutes before slicing.

To cut, use a sharp knife. For clean cuts, wipe the knife with a hot, damp cloth between each slice. Cut into 9 or 16 squares. Store the Nanaimo bars in an airtight container in the refrigerator.