Prep dry ingredients. In a large bowl combine the flour, cocoa powder, baking soda, corn starch, salt, chocolate chips, and peanut butter chips. Mix well and set aside.

Mix wet ingredients. In a separate bowl, stir together the sourdough starter (or discard), melted butter, brown sugar, white sugar, egg + egg yolk, and vanilla extract until the mixture resembles a runny caramel.

Combine wet + dry. Pour the wet mixture into the dry ingredients. Stir until just combined to form the cookie dough. Be careful not to over‑mix.

Chill the dough. Cover the dough bowl with plastic wrap and refrigerate for 12‑24 hours. This resting period helps flavor development and prevents excess spreading.

Preheat & prepare baking sheets. When ready to bake, preheat oven to 165 °C (≈ 330 °F). Line cookie sheets with parchment paper.

Portion & top. Use a cookie scoop or large spoon (~55 g per ball) to shape dough balls. Roll each ball in Halloween sprinkles. Place onto baking sheets. Add “eyes” or extra chocolate chips by gently pressing into each dough ball.

Bake. Bake in the preheated oven for 12‑13 minutes. The tops should look golden, but the centers should still look somewhat molten (not fully set).

Cool. Let cookies cool on the tray for about 10 minutes — they’ll firm up during this time. Then transfer to a wire rack to cool completely.