Preheat oven to 350°F (175°C). Grease an 8-inch square pan.

Melt butter for base in a saucepan.

Stir in sugar until dissolved.

Sift cocoa powder into the butter mixture and blend well.

Whisk egg and vanilla extract together.

Add egg mixture to the cocoa mixture and blend thoroughly.

Blend in chocolate cookie crumbs, coconut, and minced walnuts until combined.

Press the mixture firmly into the prepared pan.

Bake for 10 minutes.

Let cool for 10 minutes, then refrigerate for 20 minutes.

For the filling, beat butter until smooth.

Beat in custard powder.

Heat milk with instant coffee powder to dissolve. Let cool to room temperature.

Add milk mixture to butter mixture and stir in sifted icing sugar, a cup at a time, until smooth.

Spread the filling evenly over the cooled base.

Refrigerate while preparing the topping.

For the topping, melt chocolate and butter over a pot of simmering water (double boiler).

Pour melted chocolate over the filling and spread evenly.

Refrigerate for 30 minutes, or until set.

To serve, slice into 1-inch squares using a warm, dry knife.