In a medium saucepan, melt 1 cup of unsalted butter over medium heat. Stir frequently until the butter begins to foam, then watch for it to turn a golden brown and smell nutty, about 5-7 minutes. Once browned, remove from heat and let it cool to room temperature.

In a large bowl, combine the cooled brown butter with ¾ cup of dark brown sugar and ½ cup of white sugar. Beat on medium speed until smooth and creamy—about 2 minutes. Add 1 large egg, 1 egg yolk, and 2 teaspoons of vanilla extract, mixing until fully incorporated.

In a separate bowl, whisk together 2 cups of all-purpose flour, 1 tablespoon of espresso powder, ½ teaspoon of salt, and 1 teaspoon of baking soda. Ensure there are no lumps.

Slowly add the dry ingredient mixture into the wet ingredients, mixing on low speed until just combined. Gently fold in 1 cup of chopped dark chocolate using a spatula.

Transfer the cookie dough to a clean bowl, cover with plastic wrap, and refrigerate for at least 30 minutes. For best flavor, chill overnight.

Preheat oven to 350°F (175°C). Line a baking tray with parchment paper. Scoop rounded tablespoons of dough onto the tray, leaving 2 inches between each cookie. Bake for 11-13 minutes until edges are golden and centers are slightly gooey.

Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.