Combine Weet-Bix, flour, baking powder, cocoa and coconut in a mixing bowl.
Place sugar, golden syrup and margarine into a saucepan and simmer until sugar is dissolved.
Add vanilla essence and blend together.
Pour the liquid blend over dry ingredients and mix together.
Press into a 20cm x 30cm lined baking dish.
Bake in a moderate oven, 180°C, for 15 minutes or until firm. Remove from oven and cool.
To make icing combine icing sugar, cocoa and margarine in a bowl. Gradually add boiling water mixing until desired thickness is reached.
Spread icing over slice.