In a medium saucepan, melt the unsalted butter over medium heat. Continue to cook, stirring frequently, until it becomes golden brown and has a nutty aroma. Be careful not to burn it.
Remove from heat and transfer to a heatproof bowl. Let it cool to room temperature.
In a large bowl, whisk together the all-purpose flour, bread flour, and baking soda. Set aside.
In another large bowl, combine the cooled browned butter, granulated sugar, and dark brown sugar and beat.
Add the eggs, egg yolk, salt, and vanilla extract. Use a hand mixer to whisk until smooth and creamy.
Gradually fold the dry ingredients into the wet mixture. Stir until just combined.
Gently fold in the chopped dark chocolate and dark chocolate chips until evenly distributed.
Cover the dough with plastic wrap and chill it in the refrigerator for at least 2 hours, or preferably overnight. Let dough sit at room temperature just until it is soft enough to scoop.
Scoop out the dough using a cookie scoop or spoon (about 90 g per cookie) and place them on the parchment paper, leaving enough space between them for spreading.
Top the cookies with a pinch of flaky sea salt and a piece of chocolate. Cover lightly with plastic wrap and refrigerate for 15 minutes while the oven preheats.
Bake in a preheated oven at 175°C/350°F for 10–12 minutes, or until the edges are golden and the centers look slightly underbaked.
Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
Enjoy warm or store in an airtight container for up to a week!
Notes
Don’t Rush the Brown Butter: Browning the butter is the heart of this recipe, it adds that nutty, rich flavor to the cookies. Keep the heat on medium, stir often, and watch for the golden brown color and a toasty aroma. It happens quickly, so stay close to the stove!Let the Butter Cool: After browning, the butter is hot! If you add it to the sugar too soon, it could mess up the texture of the dough. Let it cool until it’s closer to room temperature before moving on.Chill the Dough: I know it’s tempting to bake the cookies right away, but chilling the dough makes a big difference. It helps the flavors blend and keeps the cookies from spreading too much in the oven. If you have time, overnight is best.Underbake Slightly: Take the cookies out when the edges are set but the centers look a bit soft. They’ll continue to bake on the hot tray after you take them out of the oven, leaving you with perfectly chewy cookies.Work in Batches: If your oven is small, bake the cookies in batches. Let the baking sheet cool between batches, or the dough will spread too quickly on a hot pan.