In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a separate bowl (or stand mixer) whisk together the sourdough discard, butter, eggs, sour cream, milk, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just moistened.
Fold in chocolate chips. (suggestion: also use white chocolate chips for some contrast).
Spoon batter, filling the cups until they are at the rim.
Bake for 15-20 minutes in the preheated oven. Be sure to place it on the center oven rack.
Insert a toothpick, and if it comes out clean, the muffins are done!
Place the muffin pan on a wire rack to cool completely.
Store muffins in an airtight container for 4-5 days. Or store in a freezer-safe container and freeze to keep fresh longer (3-6 months).
Notes
The yield for this recipe is about 14-16 muffins depending on how much you fill your cups. I like to fill almost to the top which will yield about 14 muffins, but filling ¾ full will give you extra.
These double chocolate muffins have a rich chocolate flavor and go great with a cup of coffee!