1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan. You can also line it with parchment paper for easier removal.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking soda, and salt. Make sure there are no lumps and the ingredients are well combined. This ensures even distribution of leavening agents.

3. In a separate medium bowl, whisk together the sourdough discard, milk, vegetable oil, large egg, and vanilla extract until smooth. The sourdough discard should be unfed and at room temperature for the best results.

4. Pour the wet ingredients into the bowl with the dry ingredients. Stir with a spatula or whisk just until no streaks of dry flour remain. Be careful not to overmix, as overmixing can lead to a tough cake. A few small lumps are perfectly fine.

5. Pour about two-thirds of the batter into your prepared baking pan and spread it evenly. Sprinkle about half of the chocolate chips (if using) over this layer. Dollop the remaining batter over the chocolate chips, then sprinkle the rest of the chocolate chips on top. Use a knife or a skewer to gently swirl the chocolate chips and the two layers of batter together, creating a beautiful marbled pattern. Don’t overdo the swirling, just a few passes are enough.

6. Place the pan in the preheated oven and bake for 25-30 minutes. The cake is done when a wooden skewer or toothpick inserted into the center comes out clean or with a few moist crumbs attached.

7. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for about 10-15 minutes before attempting to remove it. This allows the cake to set and prevents it from breaking. If you used parchment paper, you can lift it out and transfer it to the wire rack to cool completely before slicing.