With the rack in the middle position, preheat the oven to 350°F (180°C).
In a small pot, bring all of the ingredients to a boil while whisking. Simmer over medium heat for 5 minutes. Remove from the heat.
In a bowl, combine the flour, baking powder and salt.
In another bowl, whisk together the sugar, oil, vanilla and egg with an electric mixer. With the machine running on low speed, add the dry ingredients alternating with the milk. Spread the batter out in an 8-inch (20 cm) square pan. Delicately pour the sauce over the cake.
Bake for 25 minutes or until a toothpick inserted in the centre of the cake comes out clean. Let cool on a wire rack.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.