Line a 9-inch square baking pan with parchment paper, letting excess extend over the sides of pan. Spray with cooking spray.

Add the graham crackers, pecans and coconut to the bowl of a food processor and blitz until crumbs form. Set aside.

In the top of a double boiler, combine butter, cocoa, sugar, and salt. Whisk over simmering water, stirring constantly, until butter is melted. Add lightly beaten egg and cook, whisking constantly, until thickened and smooth, about 1 minute. Remove from heat; stir in graham cracker, pecan, coconut mixture. Press mixture into bottom of prepared pan. Place in the refrigerator to chill while you make the filling.

Add unsalted butter, kosher salt, powdered sugar, custard powder, vanilla and 2 tablespoons of heavy cream to the bowl of a stand mixer. Start on low to combine the ingredients, then increase to medium and mix until creamy, about 2-3 minutes.

Remove the base from the refrigerator and evenly spread the filling over the chilled layer.

Place the pan back into the refrigerator for 30 minutes or into the freezer for 15 minutes to firm up while you make the next layer.

In the top of a double boiler, combine chocolate, butter and heavy cream. Melt over simmering water, stirring constantly, until chocolate is melted and mixture is smooth. Pour over the cold buttercream and quickly spread it in an even layer using an offset spatula.

Chill the bars for 20-30 minutes, then remove from the refrigerator and use a knife to cut through the chocolate layer into the desired size bars. Then return to the refrigerator to chill another 30-60 minutes. Cut the bars using the scored marks you made (this should keep the chocolate from cracking) and enjoy! Wipe the knife clean between cuts for the cleanest slices.