In a medium saucepan, melt the butter with the sugar and cocoa, stirring often. Whisk in the egg and cook, stirring, until it thickens slightly. Remove from heat and stir in the crumbs, nuts and coconut. Press firmly into a parchment-lined 8x8-inch or 9x9-inch pan and refrigerate until cooled.

Meanwhile, beat the butter, with half the icing sugar, the custard powder and milk and cream until creamy and lump-free; beat in the remaining icing sugar, adding a little more sugar or milk if needed to make a creamy, spreadable frosting. Spread over the bottom layer and refrigerate until it firms up.

Melt the chopped chocolate (or chocolate chips) and butter in a small saucepan over low heat or in a heatproof bowl or measuring cup in the microwave, stirring until melted and smooth. Pour over the frosting layer, smoothing the top. Let sit at room temperature or refrigerate until firm before cutting into squares. Makes 16 squares.