In a medium bowl, whisk together the flour, cornmeal, baking soda, baking powder, cornstarch, and salt. Set aside.
In a large mixing bowl, whisk together the cooled browned butter, brown sugar, and granulated sugar until smooth and glossy.
Add the eggs and egg yolk, mixing until fully combined.
Whisk in the vanilla extract and active sourdough starter until smooth and there are no lumps.
Gradually fold the dry ingredients into the wet ingredients until a thick cookie dough forms. Do not overmix.
Cover the bowl tightly with plastic wrap or a lid and refrigerate overnight for the natural fermentation process. If you are using sourdough discard, you'll still want to chill the cookie dough for 1-2 hours. This helps prevent the cookies from spreading too much while baking.