Preheat the oven to 350 degrees Fahrenheit. Line a 9x11-inch baking dish with parchment paper. Sift together the flour, cornstarch, baking powder, baking soda, and salt in a large mixing bowl.
In a separate bowl, add the slightly cooled browned butter, brown sugar and white sugar and whisk together until combined. Add the eggs and vanilla and whisk until smooth.