Preheat oven to 375°F/190°C. Prepare muffin pan with liners or lightly grease.
In a large bowl, whisk together flour, sugar, cocoa, chocolate chips, baking powder, baking soda, and salt.
In a large measuring cup or medium bowl, whisk together buttermilk, oil, eggs, and vanilla.
Pour the buttermilk mixture into the flour mixture and stir until just combined. Do not over-mix. Fill the muffin cups about 2/3 full. If desired, sprinkle a few chocolate chips on top of each.
Bake for 15-17 minutes, until tops spring back lightly to the touch or a toothpick inserted in the center comes out mostly clean (unless you hit a chocolate chip!). Let sit in pan for 5 minutes, then transfer to a cooling rack to cool completely.
Notes
See blog post for buttermilk substitutions, if needed.