Place butter in a light colored pan on medium heat. Stir continuously and slowly, do not walk away.
After about 10 minutes the butter will begin to brown, instantly pour it into a metal or glass bowl to cool so the butter doesn’t burn. The container you pour it into will become VERY hot. Be cautious, and make sure the container is on a surface that is heat resistant.
Allow the brown butter to cool for about 30 minutes.
Whisk together the flour, cornstarch, salt, and baking soda, and set aside.
Add the sugar and brown sugar into the browned butter that has had time to cool. Using an electric mixer on medium speed, mix the butter and sugars together for 30 seconds.
Add the egg and vanilla into the butter and sugar mixture. Mix again on medium speed until completely incorporated.
Add the dry ingredients to the wet ingredients and stir with a large spoon just until everything is incorporated, be careful not to over mix.
Divide the dough into 15 equal balls. They will be about 1 ½ inches in diameter or use a medium cookie scoop.
Place all 15 dough balls evenly spaced apart on 2 cookie sheets lined with parchment and set in the fridge.
While the dough chills, preheat the oven to 350 degrees F.
Bake the cookies for 10 - 11 minutes, allow the cookies to cool completely on the baking sheets, then remove and serve.