Prepare the Pudding Batter: In a large bowl, combine the flour, sugar, baking powder, and cocoa powder.
Add the milk and vanilla essence to the dry ingredients and stir until well combined.
Stir in the melted butter until the batter is smooth.
Pour the batter into an ovenproof dish, spreading it out evenly.
Make the Sauce: In a separate bowl, mix together the brown sugar and cocoa powder.
Carefully pour the boiling water over the sugar-cocoa mixture and stir until the sugar dissolves.
Pour Sauce Over Batter: Gently pour the sauce mixture over the pudding batter. Do not stir.
Bake: Place the dish in the preheated oven and bake for about 40 minutes, or until the top of the pudding is firm and springy to the touch.
Serve: Allow the pudding to cool for a few minutes before serving. The pudding will have a soft, cake-like top with a rich, chocolate sauce underneath.