In a saucepan over medium heat, melt butter. Stir frequently until the butter foams, turns golden brown, and smells nutty (about 5–7 minutes). Remove from heat and allow to cool for 10 minutes.
Preheat oven to 350°F. Line a 9×9-inch baking pan with parchment paper and lightly spray with nonstick spray.
In a large mixing bowl, whisk the browned butter with brown sugar until smooth.
Add the eggs and the egg yolk one at a time, stirring well after each addition.
Stir in the vanilla extract.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, stirring until just combined.
Gently fold in most of the chocolate chips, reserving about ¼ cup for the top. The dough will be thick. Spread the dough evenly into the prepared pan and smooth the top. Then, sprinkle the reserved chocolate chips over the surface.
Bake for 33-36 minutes, or until the top is golden brown and a toothpick inserted in the center comes out with a few moist crumbs. (Do not overbake.)
Allow the bars to cool completely in the pan before cutting into squares.