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

Melt butter for base.

Stir in sugar until combined.

Sift cocoa powder into the mixture and blend well.

Whisk egg and vanilla extract together.

Add egg mixture to the butter mixture and combine.

Blend in chocolate cookie crumbs, coconut, and chopped almonds until evenly incorporated.

Press the mixture into the prepared pan.

Bake for 10 minutes.

Cool for 10 minutes, then chill for 20 minutes.

For the filling, beat butter until smooth.

Beat in custard powder until combined.

Heat milk slightly.

Add milk to the butter mixture.

Stir in icing sugar one cup at a time until smooth.

Spread the filling over the chilled base.

Chill while preparing the topping.

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

Pour the melted chocolate over the vanilla filling.

Spread to cover evenly.

Chill for 30 minutes or until set.

Slice with a hot, dry knife into 1-inch squares to serve.