Preheat oven to 350 degrees (F). Spray a 9x8 pan with non-stick spray and line with parchment paper, leave an overhang to make it easy to remove the baked brookies.
Using a hand mixer or a stand mixer, mix the cold butter, brown sugar, and granulated sugar. Beat for 4 minutes.
Add the eggs and vanilla. Beat for 1 minute.
Fold in the flour, cornstarch, baking soda, baking powder, and salt. Mix until just combined.
Fold in the mini chocolate chips.
Break chunks of cookie dough off and place them in the bottom of the pan. The chunks should be about 1/2 to 1 inch. Repeat until there are only small gaps without cookie dough.
Allow the dough to chill while you make the brownie batter.