Preheat oven to 375 degrees. Line a muffin tin with liners or spray with cooking spray.
Mix oil and sugars together until combined.
Add mashed eggs and vanilla. Mix.
Add bananas and mix until combined.
In a separate bowl, mix flour, cocoa, baking soda, baking powder, salt and cream of tarter together. Add it to the creamed mixture. Mix only until combined.
Reserve a few chocolate chips for sprinkling on top of muffins if desired. Add remaining chocolate chips to batter and fold in.
Fill muffin cups 3/4 of the way full.
Bake for 15-20 minutes.