Begin by melting the first 3 ingredients in top of double boiler. Add egg and stir to cook and thicken. Remove from heat. Stir in crumbs, coconut, and nuts. Press firmly into an ungreased 8" x 8" pan.
Second layer
Then, mix butter, cream, custard powder, and icing sugar together well. Beat until light and spread over the bottom layer. Chill in the fridge for 2 hours.
Top layer
After the first two layers are chilled, melt chocolate and butter over low heat. Finally, once cooled enough to work with, but still liquid, pour over second layer and chill in refrigerator.