TIP: Different types of cocoa powder will require different amounts of water to make a smooth mixture.
1 1/2 cups all-purpose flour
1/2 cup cake flour
1 teaspoon kosher salt
1 cup unsweetened cocoa powder
1/2 cup unsalted butter, at room temperature
1/2 cup granulated sugar
1 cup packed light brown sugar
2 large eggs, at room temperature
1 teaspoon pure vanilla extract
1/2 cup sourdough starter or unfed sourdough discard
10 ounces good-quality chocolate, roughly chopped, plus more (about 2 cups) for topping
1/2 teaspoon flaky sea salt
Whisk the all-purpose and cake flours and kosher salt together in a medium bowl. Mix the cocoa powder with 3/4 cup water until combined, adding more water if needed for a smooth mixture (see Cook's Note) and set aside.
Cream together the butter and granulated and brown sugars with an electric mixer or in a stand mixer on medium-high speed until combined and fluffy, about 2 minutes, scraping down the bowl as needed. Add the eggs one at a time while whisking. Next whisk in the vanilla.
Add the sourdough starter and whisk lightly to combine. Don't overdo it.
Add the cocoa mixture to your creamed mixture, then add half of the flour mixture and gently mix with a wooden spoon just until incorporated. Add the rest of the flour mixture. Use your spoon or a rubber spatula to gently incorporate until there is no dry flour left. Gently fold the chopped chocolate into the dough.
Cover and let this mixture rest overnight in the fridge. Alternatively, you can just let the mixture rest for 1 hour at room temperature and then bake.
The next day, or after an hour, preheat the oven to 350°F and line a sheet pan with parchment paper. Scoop 1/4 cup of dough and form into a ball, then place onto the prepared pan. Flatten a little and top with chocolate chunks and sea salt.
Bake for 15 minutes, or until deep brown. They will still be a little gooey in the middle. Allow to cool on the pan for 5 minutes before removing to a wire rack to cool completely. If you want, cut the baked cookies in half and use them to make ice cream sandwiches. I did, and they were delicious.