In a medium saucepan, melt 1 cup of unsalted butter over medium heat until deep amber and nutty aroma appears, about 5-7 minutes. Stir in 1 tablespoon of instant espresso.

In a separate bowl, whisk together 2 cups of all-purpose flour, 1 tablespoon of cornstarch, 1 teaspoon of baking soda, and ½ teaspoon of sea salt. Set aside.

In a large bowl, combine browned butter with ¾ cup of brown sugar and ½ cup of granulated sugar. Beat until smooth, then add 1 large egg, 1 egg yolk, and 2 teaspoons of vanilla extract. Mix until incorporated.

Gradually add the dry ingredients to the wet mixture and gently fold. Fold in 1 cup of chopped chocolate.

Form 2-tablespoon-sized dough balls, cover with plastic wrap, and refrigerate for at least 1 hour or up to 3 days.

Preheat oven to 350°F (175°C) 15 minutes before baking and line baking sheets with parchment paper.

Place chilled dough balls 2 inches apart on the baking sheets and bake for 8-10 minutes until edges are golden but centers are soft. Let rest for 1 minute before transferring to a wire rack.

Optional: Sprinkle warm cookies with flaky sea salt for enhanced flavor.