Begin by melting 1 1/4 cups of unsalted butter in a microwave-safe bowl; heat for 30-second intervals until smooth, then let it cool slightly for about 5 minutes.

Preheat your oven to 350°F (175°C). Line a 13x18-inch half sheet pan with parchment paper.

In a large mixing bowl, combine the cooled melted butter with 1 1/2 cups of packed light brown sugar and 1/2 cup of granulated sugar. Use a hand mixer or a whisk to mix until the ingredients are well blended and smooth, about 2-3 minutes.

Add three large eggs to the sugar mixture, one at a time, mixing thoroughly after each addition. Stir in 1 tablespoon of vanilla extract until fully incorporated.

Gradually sift in 3 1/4 cups of all-purpose flour, 1 teaspoon of cornstarch, 1 teaspoon of baking soda, and 1 teaspoon of fine salt. Gently fold the dry mixture into the wet ingredients until no flour streaks remain.

Fold in 2 cups of milk or dark chocolate chips, mixing gently.

Scoop the cookie dough onto the prepared sheet pan and use an offset spatula to evenly spread it out into an even layer. Sprinkle the reserved chocolate chips on top.

Place the sheet pan in the preheated oven and bake for 22-26 minutes, rotating the pan halfway through.

Once baked, remove the sheet pan from the oven and immediately sprinkle the top with flaky sea salt. Allow the cookies to cool completely on the pan for about 15 minutes.