Heat oven to 350°F. Line a 13×18-inch rimmed sheet pan with parchment; lightly grease the corners.
Whisk melted butter with both sugars in a large bowl until glossy and combined.
Beat in the eggs and vanilla until the mixture looks smooth and slightly thickened.
In a separate bowl, whisk flour, cornstarch, baking soda, and salt to evenly distribute.
Fold dry ingredients into the wet mixture just until no dry streaks remain.
Stir in the chocolate chips, reserving a small handful if you want to sprinkle on top.
Spread dough evenly in the prepared pan. Press gently to reach the corners; add reserved chips on top.
Bake 16–19 minutes, until golden at the edges and just set in the center.
Cool in the pan on a rack. Slice into bars once set and serve.