Line an 8x8 or 9x9 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal. Set aside.

In a medium saucepan over low to medium heat, melt the 1/2 cup butter with granulated sugar and cocoa powder, stirring constantly until smooth. Do not let it boil. Remove from heat.

Gradually whisk in the lightly beaten egg into the warm chocolate mixture, stirring quickly to prevent the egg from scrambling. Return to low heat for 1-2 minutes, stirring constantly until the mixture thickens slightly. Remove from heat and stir in 1 tsp vanilla extract.

In a large mixing bowl, combine the graham cracker crumbs, shredded coconut, and chopped pecans (if using). Pour the warm chocolate mixture over the dry ingredients and mix well until everything is evenly coated.

Press the mixture firmly and evenly into the prepared baking pan. Use the back of a spoon or your hands. Refrigerate for at least 30-60 minutes to set.

In a large mixing bowl, using an electric mixer, beat the 1/2 cup softened butter until creamy.

Gradually add the sifted powdered sugar, custard powder, 2 tbsp heavy cream, and 1 tsp vanilla extract. Beat on low speed until combined, then increase to medium-high and beat until light, fluffy, and smooth, about 3-5 minutes. Scrape down the sides of the bowl as needed.

Spread the custard filling evenly over the chilled base layer. Return the pan to the refrigerator for at least 60 minutes, or until the filling is firm to the touch.

In a small microwave-safe bowl, combine the chopped semi-sweet chocolate and 2 tbsp unsalted butter. Microwave on high in 30-second intervals, stirring after each, until melted and smooth. Be careful not to overheat.

Let the chocolate topping cool slightly, about 5-10 minutes, so it's warm but not hot. Pour it over the chilled custard layer and spread evenly with a spatula. If the custard is very cold, the chocolate may set quickly.

Return the pan to the refrigerator for at least 40-60 minutes, or until the chocolate topping is completely set. For easier cutting, you can score the chocolate with a warm knife after about 15-20 minutes of chilling, then chill fully.

Once completely set, use the parchment paper overhang to lift the entire bar out of the pan. Place on a cutting board and cut into squares or rectangles using a sharp knife dipped in hot water and wiped clean between each cut.