Put butter or margarine, cocoa and sugar into a saucepan and cook, over medium heat, until butter melts and sugar dissolves; stir occasionally.
Whisk egg and then SLOWLY whisk butter mixture into beaten egg; then put mixture back into same saucepan.
Cook, stirring frequently, for about 2 minutes or until smooth; remove from heat.
Stir in crumbs, nuts, coconut and orange rind; combine well then scrape into a 9-inch cake pan (8-inch works as well); spread evenly and even use your fingers to press down; refrigerate for 30 minutes.
Now make the middle.
With an electric mixer, beat custard powder into butter, then beat in orange juice concentrate and rind.
With your mixer on low, beat in icing sugar, a bit at a time, and the food colouring if you're using it.
Spread over the chilled base and refrigerate for 1 hour or until set.
Now make the top.
Chop chocolate coarsely and heat in a small pan, along with the butter, until melted and smooth; stir often.
Pour over the middle layer, spreading evenly; return to refrigerator for at least 30 minutes or until firm.
Once firm, cover with plastic wrap and keep refrigerated until serving.