With the rack in the middle position, preheat the oven to 350°F (180°C). Line a baking sheet with parchment paper or a silicone mat.
In a bowl, combine the flour, cocoa powder, baking soda and salt. Set aside.
In a small bowl, dissolve the cornstarch in the water. Set aside.
In another bowl, combine the brown sugar and oil with a whisk. Add the dry ingredients and the cornstarch mixture. Reserve 2 tbsp (30 ml) of the dark chocolate to garnish the cookies. With a spatula, fold the remaining chocolate in the dough.
With a 2-tbsp (30 ml) ice cream scoop, shape the dough into balls. Place them on the baking sheet. Garnish with the reserved chocolate.
Bake for about 10 minutes or until the cookies are firm on the outside but still soft in the centre. Let cool on the baking sheet.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.