Melt 1 cup of unsalted butter in a medium saucepan over medium heat. Stir continuously for about 5-7 minutes until the butter turns a deep amber color and emits a nutty fragrance. Stir in 2 tablespoons of instant espresso powder, then remove from heat and let it cool slightly.

In a separate mixing bowl, whisk together 2 1/4 cups of all-purpose flour, 1 teaspoon of cornstarch, 1 teaspoon of baking soda, and 1/2 teaspoon of sea salt.

In a large mixing bowl, combine the browned butter with 3/4 cup of brown sugar and 1/2 cup of granulated sugar. Mix on medium speed until well-combined and fluffy, about 2 minutes. Add in 1 large egg, 1 egg yolk, and 1 teaspoon of vanilla extract, and mix until light and creamy.

Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined. Fold in 1 1/2 cups of chopped chocolate.

Scoop the cookie dough into 2-tablespoon portions and place them onto a plate. Cover with plastic wrap and refrigerate for at least 1 hour.

Preheat your oven to 350°F (175°C) while the dough chills. Line two baking sheets with parchment paper. Once chilled, space the dough balls at least 2 inches apart on the sheets, then bake for 8-10 minutes.

Let the cookies rest on the baking sheet for 1 minute to set, then transfer to a wire rack to cool completely.