In a medium saucepan over medium heat, melt the butter, stirring frequently. Continue cooking until the butter turns a deep amber color and smells nutty, about 5-7 minutes. Immediately remove from heat and pour into a large mixing bowl.

Stir the instant espresso powder into the warm browned butter until fully dissolved. Set aside to cool for 10 minutes.

In a separate bowl, whisk together the flour, cornstarch, baking soda, and sea salt. Set aside.

To the cooled butter mixture, add the brown sugar and granulated sugar. Beat with an electric mixer on medium speed until well combined, about 2 minutes.

Add the whole egg, egg yolk, and vanilla extract. Beat until smooth and creamy.

Gradually add the flour mixture, mixing on low speed just until no dry streaks remain. Do not overmix.

Fold in the chopped dark chocolate with a rubber spatula until evenly distributed.

Using a cookie scoop or spoon, portion the dough into 2-tablespoon-sized balls. Place on a parchment-lined baking sheet, cover, and refrigerate for at least 1 hour, or up to 3 days for best flavor.

When ready to bake, preheat the oven to 350°F. Line two baking sheets with parchment paper.

Place the chilled cookie dough balls 2 inches apart on the prepared baking sheets. Bake for 8-10 minutes, until the edges are golden brown and the centers are still slightly soft.

Let the cookies rest on the baking sheet for 1 minute, then transfer to a wire rack to cool completely. While still warm, sprinkle with flaky sea salt if desired.