Prep your tools. Line two baking sheets with parchment paper.
Set aside. If your egg is cold, place it in warm water for 5 minutes.
Whisk the wet ingredients. In a large bowl, whisk the melted butter, brown sugar, and granulated sugar until glossy and smooth, about 30–45 seconds. Whisk in the egg and vanilla until fully combined.
Mix the dry ingredients. In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking soda, and salt to break up any lumps.
Combine. Add the dry ingredients to the wet and fold with a spatula just until no dry streaks remain.
The dough will be thick and a little sticky.
Add the chips. Fold in the semisweet and dark chocolate chips. Reserve a small handful for pressing on top later if you like a bakery-style look.
Chill the dough. Cover and refrigerate for 30–45 minutes until the dough is firm enough to scoop. This step helps prevent spreading and deepens the flavor.
Preheat the oven. Heat to 350°F (175°C) while the dough chills.
Scoop. Use a 1.5–2 tablespoon scoop to portion the dough.
Roll into balls and place them 2 inches apart on the lined baking sheets. Press a few extra chips on top for looks. If using, sprinkle lightly with flaky sea salt.
Bake. Bake for 9–11 minutes until the edges are set but the centers look slightly underbaked.
The cookies will continue to set as they cool.
Cool. Let the cookies rest on the tray for 5–7 minutes, then transfer to a wire rack to cool completely—or enjoy warm and melty.