In a large bowl in the microwave, or in a large saucepan on the stove, heat the butter until just melted. Remove from heat, add the sugars, and whisk until fully combined and creamy, 1 full minute.

Add the salt, baking powder, and baking soda, and whisk until combined.

Add the sourdough discard, egg, and vanilla and whisk until homogenous, with no streaks of discard visible.

Switch to a flexible spatula and stir in the flour and cocoa until just a few dry patches remain. Stir in the chocolate chips.

Scoop the dough into 2-tablespoon portions (about 50g each) and place them close together on a baking sheet (a quarter sheet pan works well here). Cover the baking sheet and chill for 2 to 12 hours. Alternatively, cover the bowl with the cookie dough before scooping and chill for 2 to 12 hours. The dough will need to temper at room temperature for 15 to 20 minutes before scooping.

Once the dough has chilled, preheat the oven to 350°F with racks in the upper and lower thirds. Line two baking sheets with parchment.

Scoop the cookies into 2-tablespoon portions (about 50g each) and distribute evenly among the prepared baking sheets (a 2-Tablespoon Scoop works well here).

Bake for 14 to 16 minutes, rotating top to bottom and front to back halfway through, until the cookies look matte on the top; they will still feel a bit soft but will firm up as they cool.

Let the cookies cool completely on the baking sheets before serving. Store leftover sourdough discard cookies in an airtight container at room temperature for several days.