Preheat your oven to 425°F (218°C). Position the rack in the center of the oven for even heat circulation. Line a muffin tin with paper liners or use parchment muffin cups for a bakery-style look.

In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.

In a separate bowl, combine the oil, sugar, eggs, sour cream, milk, vanilla extract, and sourdough starter. Use a whisk to mix until smooth.

Pour the wet mixture into the bowl with the dry ingredients. Gently fold them together using a spatula or wooden spoon.

Fold in the chocolate chips evenly.

Using an ice cream scoop or large spoon, fill each muffin liner to the top.

Bake the muffins at 425°F (218°C) for 5 minutes, then lower the heat to 350°F (176°C) and continue baking for 15–18 minutes.

Once baked, remove the tray from the oven and allow the muffins to cool completely in the tin before removing.