Preheat oven to 350°F (177°C). Line a 13x9 inch baking pan with parchment paper leaving some extra to hang off the sides. Set aside.
In a medium mixing bowl, whisk together the dry ingredients: flour, cornstarch, baking powder and salt. Set aside.
In a stand mixer or using a hand-held mixer, cream together the butter and both sugars on medium speed (4) for 1 minute until light and fluffy. Scrape down the sides of the bowl.
Add the eggs in one at a time on low speed and increase to medium speed to make sure they are incorporated.
Add the vanilla. Scrape down the sides of the bowl.
On low speed (1), slowly add the dry ingredients a bit at time until it is incorporated.
Scoop the dough into the baking pan and using a greased spoon or your fingers to gently press the dough into the pan.
Bake for 17-20 minutes. This will greatly depend on your oven. Be sure to watch it and not overcook it. When you start to see any color on the sides of the pan, it is time to remove it. It will continue baking in the pan.
Let it cool completely on a wire cooling rack before frosting the bars.