Preheat the oven to 180 °C / 350°F. Line a baking sheet with parchment paper. Set aside.
In a bowl, whisk together the flour, cornstarch, baking soda, and salt until combined. Set aside.
In another bowl, whisk together the melted butter with the sugars until creamy.
Add the egg & whisk again for a few seconds, until it's well incorporated.
Add the flour mixture and mix until combined. * If the dough is too wet, add a little bit more flour, one tablespoon at a time, but don't overmix.
Fold in the chocolate chunks, using a spatula or wooden spoon (or by hand), until they are evenly combined.
Using a cookie scoop or a teaspoon, form dough balls that are about 30 g (1.1 oz) each. Place them on a baking sheet, about 2 inches (5 cm) apart.
Bake for about 10-15 minutes, or until the edges are golden brown. The centers may look slightly undercooked.
Remove the cookies from the oven & immediately press some extra chocolate chunks on top of the cookies, if you like.
Allow the cookies to cool on the baking sheet for about 10 minutes or DEVOUR them while they’re still nice and warm!
Transfer the cookies to a wire rack to cool completely.
Store the cookies in an airtight container for up to a week.