1. Make the 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 ungreased 8" x 8" pan.
2. Make the middle layer: Cream butter, cream, custard powder, and icing sugar together well. Beat until light. With an offset spatula, spread over bottom layer in pan.
3. Make top layer: Melt chocolate and butter over low heat. Cool. Once cool but still liquid, pour over second layer and chill in refrigerator, using offset spatula to distribute layer evenly. Once hardened, cut into small bars