Brown the butter
Melt butter in a saucepan over medium heat.
Continue cooking, stirring frequently, until it turns golden brown and smells nutty.
Immediately remove from heat and let cool for 10–15 minutes (it should be warm, not hot).
Mix wet ingredients
In a large bowl, whisk together cooled brown butter, brown sugar, and granulated sugar.
Add eggs, vanilla extract, and milk. Mix until smooth and glossy.
Combine dry ingredients
In a separate bowl, whisk flour, cornstarch, baking soda, and salt.
Make the dough
Gradually mix dry ingredients into wet ingredients.
Stir until just combined (do not overmix).
Add chocolate
Bake
Preheat oven to 350°F (175°C).
Scoop dough into balls and place on a lined baking sheet.
Bake for 12 minutes, until edges are golden but centers still soft.
Finish
Let cool on tray for 5–10 minutes before transferring