Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift out the bars later. Lightly grease the exposed sides if necessary.

In a medium bowl, combine the gluten-free graham cracker crumbs, shredded coconut, almond flour, and cocoa powder. Mix well and set aside.

In a large saucepan over medium-low heat, melt the 1/2 cup butter and granulated sugar, stirring until smooth and the sugar is dissolved. Remove from heat.

Whisk the lightly beaten egg with the vanilla extract. Slowly temper the egg mixture into the warm butter mixture, whisking constantly to prevent the egg from scrambling. Return to very low heat for 1-2 minutes, stirring continuously until the mixture thickens slightly.

Remove from heat and stir in the dry crumb mixture until well combined. Press the mixture evenly and firmly into the bottom of the prepared pan. Use the back of a spoon or a flat bottomed glass to ensure a compact layer. Place the pan in the refrigerator to chill for at least 30 minutes while you prepare the filling.

In a large mixing bowl, using an electric mixer, beat the 1/2 cup softened butter until creamy. Gradually add the sifted confectioners' sugar, beating until light and fluffy.

In a small separate bowl, whisk together the gluten-free custard powder and milk/cream until smooth. Add this mixture to the butter and sugar mixture, along with the vanilla extract.

Beat on medium speed until the filling is smooth, pale, and fluffy, scraping down the sides of the bowl as needed. If the filling seems too thick, add another teaspoon of milk; if too thin, add a tablespoon more sifted confectioners' sugar.

Carefully spread the custard filling evenly over the chilled crumb base. Return the pan to the refrigerator to chill for another 30 minutes, or until the filling is firm to the touch.

In a double boiler or a microwave-safe bowl, melt the chopped semi-sweet chocolate and 2 tbsp butter together. Stir occasionally until completely smooth and glossy.

Allow the chocolate mixture to cool slightly (about 5-10 minutes) to prevent it from melting the custard layer when poured, but ensure it's still pourable.

Pour the cooled chocolate ganache over the chilled custard layer, spreading it gently and evenly with a spatula. Return the pan to the refrigerator for at least 1 hour, or until the chocolate topping is completely set and firm.

Once completely chilled and firm, use the parchment paper overhangs to lift the bars out of the pan onto a cutting board. Using a sharp, hot knife (dip in hot water and wipe dry between cuts), trim the edges and then cut the bars into desired squares or rectangles. For clean cuts, wipe the knife clean after each slice.

Store Nanaimo bars in an airtight container in the refrigerator for up to 5-7 days.