Brown Butter
Melt butter over medium heat until golden brown and nutty, 5–7 minutes.
Cool 10 minutes.
Prepare Dough
Whisk together flour, baking soda, cornstarch, and salt.
Beat cooled brown butter, granulated sugar, and brown sugar until light and fluffy, 2–3 minutes.
Add egg, then egg yolk. Mix.
Add vanilla extract and milk. Mix until smooth and glossy.
Gradually add dry ingredients to wet ingredients. Mix until soft dough forms. Do not overmix.
Fold in chopped chocolate.
Bake Cookies
Preheat oven to 350°F. Line baking sheets with parchment paper.
Portion dough into balls using a cookie scoop.
Press two balls together to form one large ball. Roll into smooth balls.
Place 4 dough balls on each baking sheet, spaced apart.
Chill one baking sheet for 15 minutes (or 25 minutes in fridge). Keep others in fridge.
Bake chilled sheet for 10 minutes until puffed and slightly set.
Bang baking sheet on counter three times.
Bake 3–4 minutes more until edges are golden brown and centers are pale.
Cool completely on pan.
