In a large bowl combine the dry ingredients: flour, baking soda, cornflour and salt. Set aside.
In another bowl using a wooden spoon mix the melted butter, light brown sugar and granulated sugar. Whisk until there are no brown sugar lumps remaining.
Add the egg and the egg yolk. Mix with a wooden spoon until combined. Add in the vanilla extract.
Finally add the chopped chocolate chunks and mix until just combined. The batter will be soft and thick.
Cover the dough and place it in the refrigerator for at least 2 hours.
After 2 hours take out the dough and let it come to room temperature. You can even put it in the microwave for about 5-10 seconds to just slightly soften it. But be careful not to soften the dough too much.
Preheat the oven to 190 C (375 F).
Roll the dough into balls. Each cookie should be made from about 3 tablespoons dough.
Line two baking sheets with parchment paper, and place 8 cookie dough balls on each baking sheet.
Bake for about 10-11 minutes, or until the edges are light golden. To check if the cookie is done place a butter knife just under the edge of a cookie, if it lifts easily the cookie is done.
The cookies will look very under done, but do not over bake. Let the cookies rest on the baking sheet for 10 minutes. They will continue baking from the residual heat of the baking sheet. After 10 minutes place them on a cooling rack to cool completely.
Enjoy with a cold glass of milk!