Start by preheating your oven to 350°F (175°C) and lining a baking sheet with parchment paper. Since this recipe doesn’t require chilling, having the oven ready ensures the cookies bake as soon as the dough is mixed.
In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, cornstarch, and salt. Set this dry mixture aside while you prepare the wet ingredients.
In a large mixing bowl, combine the cooled melted butter, sourdough discard, brown sugar, granulated sugar, and vanilla extract. Whisk until the mixture is smooth and well combined. If your brown sugar has clumps, it’s best to sift it before adding, as this makes mixing easier.
Add the dry ingredients into the wet mixture, stirring until just combined. Avoid overmixing, once you no longer see dry flour, stop stirring. Overmixing can lead to tough cookies instead of the soft, chewy texture we’re aiming for. Next, fold in about ¾ of the chocolate chips, reserving the rest for topping. The dough will be thicker than traditional cookie dough, so using your hands to incorporate the chocolate chips may help.
Divide the dough into nine equal pieces and roll each portion into a ball. Place them on the prepared baking sheet, ensuring at least one inch of space between each cookie. Flatten each dough ball to about half its height. Press the remaining chocolate chips into the tops of the cookies before baking.
Bake the cookies on the middle rack for 11 to 13 minutes. They may still look soft when they come out of the oven, but that’s exactly how they should be. Let them sit on the baking sheet for five minutes, allowing them to set before transferring them to a wire rack to cool completely. This resting period is essential, as the cookies will firm up while maintaining their chewy centers.