Preheat the oven to 400℉ and line muffin tins with cupcake liners, if using.
In a large bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Add in the chocolate chips and give it a toss, then set aside.
Mash the bananas, breaking them down, but leaving them slightly chunky.
In a separate bowl, whisk together the egg, sugar, vanilla, and canola oil vigorously for about 30 seconds. Add the bananas and mix, then fold in the sour cream until it comes together.
Add the wet ingredients to the dry ingredients and fold together using a rubber spatula or large wooden spoon. As soon as the mixture comes together, stop mixing (or else your muffins will get tough). The batter is very thick, so work the mixture together slowly.
Spray the liners with cooking spray, then divide the batter evenly between the muffin liners, filling each liner to the top. Toss a few extra chocolate chips on top of each muffin.
Bake for 14 - 17 minutes. Test the center with a toothpick - it should come out clean with no wet batter.
Allow the muffins to cool in the pan for a few minutes, then transfer to a wire rack to finish cooling.