Bottom Layer: Melt the butter, sugar and cocoa in a medium saucepan over medium heat. Whisk in the egg and stir to cook and thicken. Remove from heat and stir in the crumbs, nuts and coconut. Press firmly into a parchment-lined 8"x8" or 9"x9" pan and cool completely - refrigerate, even.
Middle layer: Whisk the custard powder, sugar and cream and/or milk in a large saucepan over medium-high heat, stirring constantly until the mixture bubbles and thickens. Cool completely, with a piece of parchment or plastic wrap pressed onto the surface to prevent a skin from forming. Refrigerate until well chilled, and then freeze in an ice cream machine according to the manufacturer's directions. While it's still soft, spread over the base, cover and freeze until firm.
Top Layer: Melt the chocolate and butter in a small saucepan over medium-low heat or in a bowl in the microwave, stirring every 20 seconds or so until smooth. Pour over the ice cream layer - it should set quickly. Return to the freezer or cut immediately into bars. Makes about 12 large ice cream sandwiches.