Brown Butter: Place the butter in a small saucepan over medium heat, stirring occasionally, until it starts to melt slightly and foam around. Continue to stir until you see the milk solids turn a toasty brown. Whisk in 2 tablespoons of milk until combined and pop this in the freezer for 10 minutes to cool down.
When the brown butter is solid but soft, add the two kinds of sugar (granulated sugar and brown sugar) to the bowl of a stand mixer. or whisk until creamy, airy and fluffy, about 8-10 minutes. Add the egg and vanilla and mix again for 2-3 minutes.
Dry Ingredients: In a separate bowl, whisk together all purpose flour, cornstarch, sea salt salt, baking soda.
Fold and mix the oats and chocolate chips to the dry ingredients, Add them to the wet ingredients in two batches, mixing on low until just combined. Scoop the cookie dough into balls. Chill these in the fridge. In the meanwhile preheat an oven to 375℉ And set the oven rack to the top.
Line parchment paper on a baking sheet, bake the cookies for 9-10 minutes on the top rack. After 10 minutes, check a few times and take the cookies out when they are just about golden brown.
They may still look slightly underdone in the center, that is totally fine. Allow them to cool for 20-30 minutes before serving.