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

In a large mixing bowl, whisk together melted unsalted butter, light brown sugar, and granulated sugar until smooth and glossy.

Add in large egg and vanilla extract, stirring until well mixed.

In a separate bowl, combine all-purpose flour, unsweetened cocoa powder, cornstarch, baking soda, and fine sea salt. Whisk together until mixed.

Fold the dry ingredients into the wet mixture using a spatula until just combined.

Gently fold in semisweet chocolate chips and dark chocolate chips, reserving a few for topping.

Cover the bowl with plastic wrap and chill in the refrigerator for 30-45 minutes.

Scoop out the chilled dough, rolling into balls and placing them on the prepared baking sheets, spacing them about 2 inches apart.

Press reserved chocolate chips on top of each cookie and bake for 9-11 minutes.

Allow cookies to cool on the baking sheets for 5-7 minutes before transferring to a wire rack.