Melt the butter carefully to avoid burning it.

In a large bowl, whisk together the flour, baking soda, cornstarch, and salt.

In a smaller bowl, beat the two sugars and melted butter with a whisk (by hand) until there are no large lumps of brown sugar.

In the second bowl, beat the egg, then the yolk, and finally the vanilla.

Pour the wet ingredients into the flour mixture and mix with a wooden spoon or a silicone spatula. The dough will be soft but thick.

Add the chocolate chunks and try to incorporate them into the dough as much as possible.

Cover the dough well and refrigerate for at least 3 hours. You can also prepare it the day before baking. This helps prevent the cookies from spreading too much. If left in the fridge for more than 4 hours, let the cold dough sit out for about 10 minutes before handling.

Take about 2 tablespoons of dough or use an ice cream scoop (about 32-35 grams). Form balls and place them on baking sheets lined with parchment paper, spaced apart as they will spread.

Bake for 12 minutes at 325°F in a preheated oven. The cookies are ready when the edges turn golden. For crispier cookies, leave them in a bit longer. When you take them out of the oven, they will still be very soft; let them cool for a few minutes and enjoy!