In a large bowl, stir together the sourdough discard, cornmeal, flour, and milk until well combined.

Cover the bowl and let the batter rest at room temperature for 4–8 hours. You can also refrigerate it overnight if that works better for your schedule.

When you're ready to bake, preheat the oven to 350°F.

In a small bowl, whisk together the melted butter, eggs, and maple syrup. Add this mixture into the rested batter.

Add the baking powder, salt, and baking soda to the batter and stir until everything is fully combined.

Grease or line a muffin pan and divide the batter evenly between the cups, filling them about 3/4 of the way full.

Bake for 15–18 minutes, until the tops are golden and a toothpick inserted in the center comes out clean.

Let the muffins cool for a few minutes before serving.