In a medium bowl, combine the melted butter, graham cracker crumbs, shredded coconut, and chopped walnuts (if using).
Press the mixture evenly into the bottom of a greased 9x13-inch baking pan.
Refrigerate while preparing the next layers.
In a large bowl, beat the softened butter until creamy.
Gradually add the powdered sugar and vanilla custard powder, mixing until smooth.
Add the milk and continue to mix until well combined.
Spread the custard mixture evenly over the chilled base layer.
Refrigerate again while preparing the chocolate layer.
In a small saucepan over low heat, melt the semi-sweet chocolate and butter, stirring constantly until smooth.
Pour the melted chocolate over the custard layer, spreading it evenly with a spatula.
Refrigerate the bars for at least 2 hours or until the chocolate is set and firm.
Once chilled, cut into squares and serve.
Enjoy your meal!