Brown the butter. Melt the butter in a light-colored saucepan over medium heat. Swirl and stir as it foams; watch for amber color and a toasty, nutty smell. When brown specks form on the bottom, remove from heat and transfer to a heatproof bowl to stop cooking.

Add the espresso. Stir the instant espresso into the warm brown butter until it dissolves. Let the mixture cool slightly.

Whisk the dry mix. In a separate bowl combine flour, cornstarch, baking soda, and salt. Set aside.

Combine sugars and brown butter. Add both sugars to the brown butter and whisk until smooth and glossy.

Incorporate eggs and vanilla. Beat in the whole egg, the extra yolk, and vanilla until the mixture looks silky.

Fold in dry ingredients. Gently fold the flour mixture into the wet base just until no streaks remain. Avoid overmixing.

Add chocolate. Fold in the chopped chocolate, leaving a few pieces to press on top of the dough balls if desired.

Chill the dough. Portion dough into roughly 2-tablespoon (≈1 oz) scoops. Cover and refrigerate for at least 1 hour — overnight or up to 72 hours is better for flavor and texture. Chilling is key for thick, chewy cookies.

Bake. Preheat oven to 350°F (175°C). Line baking sheets with parchment. Place dough 2 inches apart and bake 8–10 minutes, until the edges are golden and centers still look slightly soft.

Finish & cool. Let cookies rest on the sheet 1 minute, then transfer to a wire rack to cool. Sprinkle flaky sea salt on tops while warm.