Preheat oern to 180 degrees C, and grease a 1.5 litre ovenproof dish.
In a medium saucepan, melt the butter, milk and vanilla essence. Remove from heat and whisk in caster sugar, then whisk in the three flours, baking powder, half the cocoa powder and rind. Pour into baking dish, smoothing the surface.
Mix the remaining cocoa powder and brown sugar then evenly cover the top of the flour mixture. Gently pour the boiling water over the top. Bake in preheated oven for 40 minutes. Remove from the oven and allow to stand for 5 minutes before serving.