Melt the butter and half of the chopped dark chocolate together in a double boiler or microwave-safe bowl, stirring until smooth and glossy. Let it cool slightly, until just warm and fragrant.
In a separate bowl, whisk the eggs and dark brown sugar until the mixture is pale, thick, and slightly frothy—about 2 minutes. This adds lightness and helps incorporate air into the batter.
Pour the slightly cooled chocolate-butter mixture into the egg-sugar mixture, stirring gently to combine. Add the vanilla extract and mix until smooth and homogeneous.
Sift the cocoa powder and flour into the wet mixture to prevent lumps, then gently fold everything together with a spatula until just combined. The batter will be thick and fudgy—don't overmix to keep it tender.
Fold in the remaining chopped chocolate chunks, distributing them evenly throughout the batter. This creates gooey pockets of melted chocolate inside each cookie.
Spoon heaping tablespoons of dough onto a parchment-lined baking sheet, spacing them about 2 inches apart. Slightly flatten each mound to encourage even spreading.
Bake the cookies in a preheated oven at 180°C (350°F) for 10 to 12 minutes. They’ll crack on top and the edges will be set, but the centers should still look slightly soft and glossy.
Remove the baking sheet from the oven and let the cookies rest for 5 minutes. They will continue to set as they cool, developing that perfect crispy edge and gooey center.
Transfer the cookies to a wire rack and cool for at least 10 minutes. This allows the centers to firm up slightly while remaining irresistibly soft and melty inside.
Serve these cookies warm or at room temperature, enjoying the contrast of crisp edges and rich, gooey centers. If desired, sprinkle with a touch of sea salt for extra flavor.
