Preheat the oven to 180 C / Gas 4. Grease a medium sized deep glass baking dish.
In a mixing bowl combine 175g caster sugar, flour, 30g cocoa powder, baking powder and salt. Gradually beat in the milk, butter and vanilla and mix until smooth. Pour the mixture into the prepared baking dish.
In a separate bowl combine the brown sugar, remaining caster sugar and cocoa powder and the water. Pour the mixture into the centre of the cake mixture, using a spoon to break the flow - do not stir the two mixtures together.
Bake in the preheated oven for 35 to 40 minutes until the outside of the cake is firm but the centre is gooey. Remove from the oven and allow to stand for 15 minutes before serving.