Start by lining two baking sheets with parchment paper.

In a large bowl, combine melted unsalted butter, light brown sugar, and granulated sugar until glossy. Add in the egg and vanilla extract, mixing well.

In another bowl, whisk together all-purpose flour, cocoa powder, cornstarch (if using), baking soda, and fine sea salt.

Gently fold the dry mixture into the wet mixture until just combined.

Carefully fold in the semisweet chocolate chips, saving a few for later.

Cover the bowl with plastic wrap and refrigerate for 30 to 45 minutes.

Preheat your oven to 350°F (175°C).

Using a cookie scoop, portion out the dough into balls, placing them 2 inches apart on the lined sheets.

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

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

Optional: Sprinkle with flaky sea salt before serving.