Preheat oven to 170°C / 150°C fan. Generously butter a medium-large baking dish. Boil the kettle.
In a bowl, whisk together flour, baking powder, dark brown sugar, cocoa powder, and salt.
Add milk, melted butter, eggs, and vanilla. Whisk until smooth and pour into the prepared dish. Add the chocolate chips on top of the batter.
Mix topping ingredients together and sprinkle evenly over batter. Using the back of a spoon, gently pour the boiling water across the surface.
Bake for 33–35 minutes until the top is set and crisp.
Leave to stand for 3 minutes before serving, the sauce will thicken slightly beneath the sponge. Serve warm with cream or ice cream.
