Preheat the oven to 325°F(165C°). Line baking sheet with parchment paper or silicone baking mat. Set aside.

Combine the flour, baking soda, cocoa powder and cornstarch in a medium bowl and set aside.

Put the butter in a large bowl and beat on medium speed for 1 to 2 minutes until it is soft with a hand-held mixer or with paddle attachment if using a stand-mixer. Add the brown and white sugar and cream on medium speed until light and fluffy. Add the egg and mix well. Then add the vanilla and mix. Stop the machine and scrape down the sides and bottom of the bowl with a spatula as needed. Add flour mixture in 3 batches until it is totally incorporated. Stir in the chocolate and mix evenly with a spatula or with your hand.

Roll the dough into 40 gram (2 tablespoons) balls and place them on the baking sheet. Leave enough space between the balls about 2 inches (5cm).