In a bowl, mix together the dry ingredients (flour, cornstarch, baking soda, baking powder, and salt). Set aside.
In a separate large mixing bowl, beat the softened butter with the light brown sugar and granulated sugar until light and fluffy. Beat in egg, egg yolk and vanilla extract.
Stir in the dry ingredients. Don’t overmix.
Finally, fold in the chocolate chips. Cover the bowl with plastic wrap and place in the refrigerator for at least 2 hours or overnight.
When ready to bake, remove the dough from the refrigerator and let it soften for 10 minutes.
Preheat the oven to 325F. Line a baking sheet with parchment paper.
Using a cookie scoop or a spoon, scoop about 2 tbsp of the dough and roll with your hands into a tall ball. Place on the prepared baking sheet, leaving 2 inches between each cookie. Work in batches or use two baking sheets.
Bake in the preheated oven for 10-12 minutes or until the edges are golden brown and the center is still soft. Remove from the oven and let cool on the baking sheet for 5 minutes. Then, transfer to a cooling rack. Enjoy!