Make sure to carefully follow each step of the instructions for the best results :
- Brown the butter.
Cut the butter into pieces and place in a light-colored saucepan or skillet over medium heat. Melt it while stirring occasionally, then continue cooking as it foams. Once the foam subsides, golden-brown specks will appear on the bottom and the butter will turn amber with a rich, nutty aroma. Immediately pour it into a large mixing bowl, scraping every bit of the browned bits from the pan. Using a light-colored pan makes it easier to see the color change clearly, and removing the butter from the heat the moment it turns golden prevents it from burning. - Cool the brown butter.
Let the browned butter cool to room temperature, about 20 to 30 minutes. You can speed this up by placing the bowl in the refrigerator for 10 to 15 minutes. The butter must be cooled before adding the sugar and eggs or it will melt them, causing a greasy dough that spreads too thin in the oven. - Mix the sugars and eggs.
Add both sugars to the cooled brown butter and whisk vigorously for about 1 to 2 minutes until well combined and slightly lighter in color. Add the eggs and vanilla extract and whisk again for another minute until the mixture is smooth and glossy. Whisking the eggs and sugar into the cooled brown butter builds structure and helps the cookies hold their shape while baking. - Add the dry ingredients.
In a separate bowl, whisk together the flour, baking soda, cornstarch, and salt. Add the dry ingredients to the butter mixture and stir gently with a spatula until just combined and no dry streaks remain. Avoid overmixing, which can develop too much gluten and make the cookies tough. - Fold in the chocolate.
Stir in the chocolate chips or chopped chocolate until evenly distributed throughout the dough. Using chopped chocolate instead of chips creates uneven puddles and shards that melt beautifully throughout each cookie. - Chill the dough.
Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes and up to 72 hours. Chilling firms the dough, prevents excessive spreading, and allows the flour to hydrate fully, which deepens the flavor and improves the texture significantly. - Bake.
Preheat the oven to 375°F and line baking sheets with parchment paper. Scoop the chilled dough into balls using a large 3-tablespoon cookie scoop and place them at least 2 inches apart on the sheet. Bake for 11 to 13 minutes until the edges are golden and set but the centers still look slightly underdone and soft. The cookies will firm up as they cool. - Finish with sea salt.
Immediately after removing the cookies from the oven, sprinkle each one with a pinch of flaky sea salt. Let them cool on the baking sheet for 5 minutes before transferring to a wire rack. Flaky salt added right out of the oven adheres to the surface and amplifies every layer of flavor.