Preheat oven to 400ºF
In large bowl using a mixing spoon, mix sugar, sour cream, oil, eggs and vanilla.
Sift in flour, baking soda, baking powder, salt and cocoa. Gently stir until just barely mixed. Batter does not need to be completely smooth. Stir in chocolate chips.
Scoop batter into muffin pans prepared with liners. Divide batter between 6 jumbo muffin tins or 12 standard muffin tins. Top muffins with a few extra chocolate chips, press them slightly into the batter.
Jumbo Muffins: Bake at 400ºF for 10 minutes then decrease temperature to 350ºF and bake for 15-17 more minutes.
Standard Muffins: Bake at 400ºF for 8 minutes then decrease temperature to 350ºF and bake for 8-10 more minutes.
Allow muffins to cool on a rack completely before transferring to an airtight container or bag to store.