With the rack in the middle position, preheat the oven to 400°F (200°C). Butter an 8-inch (20 cm) square glass baking dish, 2 inches (5 cm) high.
In a bowl, combine the flour and baking powder.
In another bowl, cream the sugar and butter with an electric mixer. Add the egg, milk and vanilla. Add the dry ingredients, mixing just to combine.
In the baking dish, whisk together the brown sugar, water and cream. Using a spoon, gently scoop the batter on top of the brown sugar mixture. Bake for 35 minutes or until a toothpick inserted in the centre comes out clean. Let cool 15 minutes before serving.