Line a baking sheet with silicone mat or parchment paper. Set aside.
In a bowl sift together all purpose flour, baking soda, cornstarch and salt. Set aside.
Beat together butter, oil and both the sugars until creamy.
Add in the vanilla, egg, egg yolk and beat till well combined.
Gradually beat in the flour mixture. [Add it in 3 parts and not dump it all in one go].
Stir in the chocolate chips.
Chill the mixture for 3-4 hours or overnight.
Drop heaping tall tablespoons of dough onto the prepared baking sheet. I used around 1.55 oz of dough per cookie. You can push few chocolate chips on top of cookie dough for presentation purpose but that's optional.
Bake at 350 F degrees for 9-11 minutes, I baked mine for 10 minutes.
Take out of the oven and let the cookies cool down in the baking sheet for 10 minutes before transferring them on to a wire rack to cool completely.