Preheat oven to 180C/350F. Grease a 20cm x 30cm (8-inch x 12-inch) (top measurement) slice pan; line the base and long sides with baking paper, extending the paper 5cm (2 inches) over sides.
Combine butter, sugar, egg and vanilla in a medium bowl. Stir in sifted flour, baking powder and cocoa powder, then desiccated coconut. Spread mixture evenly over the base of the pan.rn
Bake for about 30 minutes.
Meanwhile, make chocolate icing. Sift icing sugar and cocoa powder into a medium bowl; add combined butter and the water and stir until icing is a spreadable consistency.
Spread hot slice with chocolate icing; sprinkle with extra coconut. Cool in pan; cut into 12 pieces.