Lightly oil a 23 cm square tin and line with clingfilm.

Melt 125g of butter.

Mix together the melted butter, sugar, cocoa powder, and lightly beaten egg.

Add biscuit crumbs, coconut, and chopped walnuts.

Combine thoroughly.

Press the mixture into the prepared tin very firmly.

Chill in the refrigerator for at least 1 hour to set the base.

For the filling, cream together the 50g of butter, custard powder, and vanilla essence until smooth.

Add the milk and sifted icing sugar alternately, about one-third at a time, mixing until smooth after each addition.

Spread the custard cream mixture evenly over the chilled base.

Return the tin to the refrigerator and chill again for a further hour.

For the topping, melt the dark chocolate together with the 1 tablespoon of butter.

Stir together until combined well and the chocolate is glossy.

Pour the melted chocolate over the custard cream filling.

Spread quickly to cover the entire base and filling with a thin layer of chocolate.

Leave for 5-10 minutes until the chocolate starts to set.

Mark the chocolate with a sharp knife into 18 squares.

Refrigerate until fully set.

Cut through the marks to divide the cake into individual squares.

Serve the Nanaimo bars chilled.