Preheat oven to 350F (180C).

Grease a 13- x -9 inch (3.5 L) metal cake pan and line with parchment paper.

In a bowl, whisk melted butter, sugar, and eggs until combined.

Stir in chocolate cookie crumbs and sweetened flaked coconut until evenly distributed.

Press the mixture firmly into the bottom of the prepared pan.

Bake in the preheated oven for approximately 20 minutes, or until the base is firm.

Remove from oven and let cool completely in the pan on a wire rack.

For the filling, combine melted butter, milk, custard powder, and peppermint extract in a bowl.

Beat in icing sugar until the filling is smooth and creamy.

Add a drop of green food coloring and mix until desired shade is achieved.

Spread the custard filling evenly over the cooled base.

Refrigerate for about 45 minutes, or until the filling is firm.

For the topping, melt chopped semisweet chocolate and butter in a heatproof bowl set over a saucepan of simmering water, stirring occasionally until smooth.

Spread the melted chocolate evenly over the firm filling.

Refrigerate until the chocolate is completely set.

Cut into 40 bars and serve.