Preheat the oven to 170°C, line a 9x13 inch baking tray with oil or butter. Set aside.
In a large mixing bowl add the brown sugar, flour, cocoa powder, baking powder and salt. Whisk until no lumps.
Add the vanilla, milk, egg, melted butter and whisk together until no lumps. Add the dark chocolate into the batter and mix until distributed evenly.
Transfer to the lined baking tray and set aside.
To make the chocolate sauce mix all ingredients together until combined and pour over the batter in the baking tin.
Bake for 28-30 mins until the top is risen, cracked, and dry and the sauce is bubbling on the sides. Remove and leave to cool for 10-15 mins before serving.