In a bowl of medium size, mix the flour, baking soda, cornstarch, and kosher salt together. Put it aside.

In another bowl of large size, use an electric hand mixer or stand mixer to cream the melted butter, dark brown sugar, and granulated white sugar until well blended and smooth.

Then add the egg, egg yolk, and vanilla extract and mix until completely incorporated.

Finally, add the dry ingredients to the wet ingredients, mixing on the lowest speed until just combined. Over-mixing will lead to maintaining a soft texture.

Wrap up dough and place it in a refrigerator for at least 30 minutes, as this will help in flavor development and keep it from spreading too much in baking. When you have time, chill even longer, at least an hour or two, because the dough will get even firmer, resulting in much thicker cookies.

Preheat your oven to 350°F (175°C) and prepare a baking sheet with parchment paper. Measure out the scooped dough onto the baking sheet using the cookie scoop or tablespoon, 2-by-2-inches apart. Bake for about 10-12 minutes or until the edges are golden brown and the centers seem slightly underdone.

Take cookies out of the oven and allow to cool on the baking sheet for 5 minutes before transferring to a wire rack. Add a pinch of flaky sea salt as the final touch to counterbalance the sweetness.