Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.

Whisk together the melted butter, light brown sugar, and granulated sugar until glossy, about 2 minutes. Add the egg and vanilla extract, whisking until smooth.

In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking soda, and salt.

Fold the dry ingredients into the wet mixture until no dry streaks remain.

Gently fold in the semisweet and dark chocolate chips, reserving some for topping.

Cover the mixing bowl with plastic wrap and refrigerate the dough for 30-45 minutes.

Scoop the dough onto the prepared baking sheets, spacing them about 2 inches apart. Press reserved chocolate chips on top and sprinkle with flaky sea salt.

Bake for 9 to 11 minutes, ensuring edges are set and the centers are slightly underbaked.

Let cool on the baking sheet for 5-7 minutes before transferring to a wire rack.