Preheat oven to 180 degrees C. Lightly grease a 9x13-inch baking dish.

Sift together the flour, cornstarch, baking soda and baking powder into a large bowl and set aside.

Cream together the butter and sugars on medium speed until very light, about 5 minutes. Add the eggs one at a time, mixing well after each addition, then add the vanilla. Reduce the mixer speed to low, gradually add the dry ingredients and mix until just combined, 5 to 10 seconds. Using a rubber spatula, fold in the chocolate chips.

Turn the dough out into the prepared baking pan and press into an even layer.

Bake until the edges cookies are light brown and set, but the center still look pale, about 15 to 20 minutes.

Allow to cool completely, then cut into squares and serve. Leftover cookie bars can be stored in an airtight container at room temperature for up to 4 days.