- With the rack in the middle position, preheat the oven to 180°C (350°F). Butter a 23 x 33-cm (9 x 13-inch) Pyrex or aluminum baking dish.
- In a saucepan, combine the brown sugar, cocoa powder and cornstarch. Add the water and cream. Bring to a boil, stirring constantly with a whisk. Add the vanilla. Set aside.
- In a small saucepan, bring the milk and cocoa powder to a boil, whisking constantly. Let cool.
- In a bowl, combine the flour, baking soda and salt. Set aside.
- In another bowl, cream the butter and sugar with an electric mixer until the mixture resembles coarse, moist sand. Add the eggs and beat until the mixture is smooth. Add the dry ingredients, alternating with the milk mixture. Scoop the batter into the baking dish.
- Gently pour the hot sauce over the batter. Bake until a toothpick inserted in the centre of the cake comes out clean, about 45 minutes. Serve warm or cold.