Preheat oven to 400° Fahrenheit (204° Celsius).
In a medium mixing bowl, whisk together the flour, cocoa powder, sugar, baking soda, and salt.
In a small mixing bowl, whisk together the sourdough discard, milk, butter, vanilla extract, and egg.
Pour the wet ingredients into the dry ingredients and stir until just moistened.
Fold in chocolate chips.
Lightly grease a muffin tin and scoop the batter into each cup, filling the cups until they are 2/3 full.
Bake for 20 minutes. Insert a toothpick, and if it comes out clean, the muffins are done!