Preheat oven to 350F/175C. Grease muffin pan cups or line them with liner papers.
In a large bowl sift together flour, cocoa, baking soda, and salt.
In a medium bowl, whisk together oil, sugar, eggs, and vanilla extract. Add mashed bananas and sour cream and mix until combined. Fold in the flour mixture just until combined. Do not overmix; the batter should be a bit lumpy. Stir in chocolate chips.
Divide mixture between muffin cups, filling about ¾ full. Bake for 15 minutes or until a toothpick inserted into the center of a muffin comes out clean. Transfer to a wire rack and let cool for 10 minutes before removing from pan.
Muffins will keep fresh and moist for 3 days at room temperature or in the fridge.