Preheat oven to 170ºC. Lightly grease and line a 20 x 30cm baking pan. Melt butter and chocolate in a medium heatproof bowl over a saucepan filled with half-simmering water (make sure the bowl does not touch the water). Use a metal spoon to stir occasionally until the chocolate melts and is smooth. Remove from heat. Place eggs, caster sugar, coconut and vanilla essence in a large bowl. Stir until well combined. Combine flour and cocoa in a separate bowl. Fold melted chocolate into egg mixture, then fold through flour mixture. Stir until just combined. Spoon mixture into prepared pan. Bake for 30-35 minutes or until just firm. Set aside to cool. To serve, slice and dust with icing sugar or cocoa.