Melt the butter. Place 2 sticks of unsalted butter in a medium saucepan over medium heat. Once the butter melts, begin whisking occasionally. After a few minutes, the butter will start to foam. This process can take about 4-7 minutes.
Brown the butter. Once the foam forms, whisk continuously, as it becomes harder to see what is happening underneath the surface. As the butter continues to cook, the milk solids will sink to the bottom of the pan. At first, these solids will be pale, but within a minute or two, they will begin to toast and turn golden brown.
Continue to brown the butter. Stir continuously to prevent the solids from burning. The butter is ready when the solids reach a deep amber color and the butter gives off a rich, nutty aroma. At this point, remove the butter from the heat to prevent burning.
Cool the butter. Carefully pour the butter into a glass measuring cup or heatproof jar, making sure to scrape in all the browned bits from the bottom of the pan. These toasted bits add depth to the flavor. Let it cool to room temperature.
Mix the sugars and butter. In a large mixing bowl, combine the cooled brown butter with white granulated sugar and packed dark brown sugar. Use an electric mixer or whisk to beat the mixture until smooth and creamy.
Add the eggs. Add two whole eggs and one egg yolk, plus the vanilla extract. Mix well until fully incorporated.
Combine the dry ingredients. In a separate bowl, whisk together all-purpose flour, cornstarch, baking soda, and sea salt.
Combine the dry and wet ingredients. Gradually add dry ingredients to the wet mixture, stirring with a whisker until just combined. Be careful not to overmix, as this could make the cookies tough. The dough will be thick and slightly sticky.
Add chocolate. Gently fold in 2 cups of semi-sweet chocolate chips, making sure they are evenly distributed throughout the dough.
Chill the dough. Cover the cookie dough with plastic wrap and chill it in the refrigerator for at least 30 minutes. This step helps solidify the butter, ensuring the cookies don’t spread too much during baking.
Prep the oven. Preheat the oven to 350°F. Line two baking sheets with parchment paper to prevent sticking.
Shape the cookie dough balls. Use a cookie scoop or tablespoon to portion the dough, rolling each scoop into a ball. Place the dough balls on the prepared baking sheets, leaving about 2 inches of space between them to allow for spreading.
Bake. Bake the cookies for 10-11 minutes, or until the edges are golden brown and the centers are still soft. They will continue to set as they cool. Avoid over baking to keep the cookies chewy.
Remove from oven and cool. As soon as the cookies come out of the oven, sprinkle each one with a pinch of flaky sea salt to enhance the flavors. Let the cookies rest on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.