Add the egg, and stir until it has cooked and thickened.
Remove from the heat, and stir in the crumbs, and nuts.
Press firmly and evenly into the pan. Chill in the fridge while making the second layer.
For the custard layer, cream together the butter, custard powder, and icing sugar, until light and fluffy.
Add whipping cream, and whip until light. Spread over first layer, and chill until firm.
For the chocolate topping, melt the butter and chocolate together over a bain marie, being careful not to overheat.
Remove bowl from the heat, and allow to cool. Once cool, but still pourable, spread over the custard layer, and chill to set.
Notes
*Bain marie is a fancy way of saying hot water bath. It is used to describe cooking items in the oven surrounded by water (to ensure even cooking), or cooking items in a bowl set over a pan of simmering water. In our case, it means the latter.