For bottom layer, melt 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 un-greased 8-inch square pan.
For middle layer, cream butter, cream, custard powder, and icing sugar together well. Beat until light. Spread over bottom layer.
For top layer, melt chocolate and butter over low heat. Cool. Once cool, but still liquid, pour over second layer and chill in refrigerator.