Prep: Preheat your oven to 180°C. Lightly grease a deep baking dish (a square dish works best for even baking).
Mix the Batter: In a large bowl, sift together the self-raising flour and cocoa powder. Stir in the caster sugar. Add the milk and melted butter, then whisk until the mixture is smooth and well combined.
Transfer: Spread the batter evenly into your prepared baking dish.
Add the Topping: In a separate small bowl, mix the brown sugar and the extra cocoa powder. Sprinkle this mixture evenly over the top of the batter in the dish.
The Secret Sauce Step: Carefully pour the boiling water over the back of a large metal spoon to cover the sugar mixture. Do this gently so you don’t disturb the batter layer. Do not stir!
Bake: Place in the oven for 35 to 40 minutes, or until the top of the cake is firm to the touch. The water and sugar will have settled at the bottom to create a rich, gooey chocolate sauce.
Serve: Let it stand for 5 minutes before serving. It is best enjoyed warm, perhaps with a scoop of vanilla ice cream or a dollop of thick cream.
