Start by browning the butter. Place the butter in a saucepan over medium heat and cook, stirring frequently. After about 8–10 minutes, it will foam, darken slightly, and develop golden brown specks at the bottom—this is your cue that it’s ready. The aroma will be nutty and rich. Immediately pour the browned butter into a large mixing bowl to stop it from cooking further. Let it cool for 20–30 minutes until it’s just warm to the touch—ideally between 90–93°F (32–34°C). It shouldn’t be hot, or it may melt the sugars too much, but also not too cool, or it will firm up when mixing.

In a separate medium bowl, whisk together the flour, baking soda, and salt until evenly combined. Set aside.

Once the browned butter has cooled, add the light brown sugar and granulated sugar to the bowl. Mix with a spatula until well combined. The mixture should resemble wet sand. (If using a stand mixer, fit it with the paddle attachment and mix on medium speed for about 20 seconds.)

Add the egg, milk, and vanilla extract to the sugar-butter mixture. Stir gently until the mixture becomes smooth and creamy. Avoid overbeating, especially if your kitchen is cool—overmixing can cause the butterfat to stiffen and make the dough tough.

Gradually add the flour mixture to the wet ingredients. Stir until the flour is mostly incorporated, then add the dark chocolate chunks and semisweet chocolate chips. Continue mixing until everything is evenly distributed, being careful not to overmix.

Cover the bowl and refrigerate the cookie dough for 1 to 4 hours. For best results, chill for at least 2 hours—this helps develop flavor and improves the cookie texture. If you’re short on time, a 1-hour chill will still work.

Preheat your oven to 375°F (190°C). Line two large baking sheets with parchment paper.

Use a 1½-ounce cookie scoop (or measure out about 2 oz / 56g per cookie) and roll the dough into smooth balls. You should get around 14 cookies. Place them on the prepared baking sheets, spacing them about 3 inches apart. If desired, press a few extra chocolate chunks into the tops for an extra-pretty finish. Do not flatten the dough balls.

Bake for 8–10 minutes, until the cookies are golden brown on top, lightly crisp around the edges, and still soft in the center. (For smaller cookies using a 1¼-ounce scoop, reduce bake time slightly.)

Allow cookies to cool on the baking sheet for about 2 minutes, then carefully transfer them to a wire rack to cool completely. While they’re still warm, sprinkle with flaky sea salt if you’d like an extra burst of flavor.