Brown the Butter: In a saucepan over medium heat, melt the butter, stirring constantly until it turns golden brown and gives off a nutty aroma. This should take about 5-7 minutes. Remove from heat and let it cool for about 10-15 minutes until it reaches room temperature.

Mix Wet Ingredients: In a large bowl, use a hand mixer or stand mixer with a paddle attachment to beat the browned butter and both sugars together on medium speed until fluffy and well combined. Add the egg, egg yolk, and vanilla extract, mixing until fully incorporated.

Mix Dry Ingredients: In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt. Slowly add the dry ingredients to the wet ingredients, mixing on low speed until just combined. The dough will be thick.

Add Chocolate: Gently fold in the semi-sweet chocolate chunks and milk chocolate chips until evenly distributed.

Form the Cookies and Chill: Scoop about 3–4 oz of dough per cookie (or adjust to your preferred size) and place the dough balls on a parchment-lined baking sheet or tray. Cover tightly with plastic wrap and chill for at least 24 hours for the best flavor (1 hour minimum if short on time). Chilling the pre-scooped dough helps the flavors develop and prevents overspreading while keeping portioning easy.

Preheat the oven to 350°F (177°C). Line a baking sheet with parchment paper or a silicone baking mat. Arrange the chilled dough balls a few inches apart on the prepared sheet. If the dough has been chilled for more than an hour, let it sit at room temperature for about 10 minutes before baking.

Bake: Bake the cookies for about 8-11 minutes, or until the edges are just turning golden. The exact time will vary depending on the size of your cookies. The centers should look soft and underbaked, as they’ll continue to cook as they cool.

Sea Salt Finish: After removing from the oven, immediately sprinkle flaky sea salt on top of each cookie. Let them cool on the baking sheet for 5 minutes before transferring to a cooling rack.

Enjoy: Serve warm or at room temperature, and enjoy the nutty, rich flavors of the brown butter with melty chocolate chunks!