Preheat the oven to 350 degrees F and line a 9x9 inch pan with parchment paper and spray it with nonstick baking spray.

In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar.

Whisk in the eggs and vanilla extract.

In a medium-sized mixing bowl whisk together all-purpose flour, cornstarch, baking powder, and kosher salt.

Fold the flour mixture into the sugar mixtures until no clumps of flour remain in the batter.

Fold in the white chocolate chips.

Pour into the prepared baking dish and bake for 25-30 minutes or until a toothpick inserted in the middle comes out clean.

Cool completely in the pan and then slice into 9 squares.

Store leftovers in an airtight container.