Preheat your oven to 350°F (175°C) to ensure even baking.
Line muffin tin with parchment paper or muffin liners to prevent sticking.
In a medium bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder, and a pinch of salt.
In a large bowl, whisk together egg, canola oil, sour cream, and vanilla extract until smooth.
Add mashed bananas to the wet mixture and mix until fully incorporated.
Gently fold the dry ingredients into the wet mixture using a rubber spatula, being careful not to overmix.
Fold in the mini chocolate chips until evenly distributed.
Scoop batter into the muffin tin, filling each cup about two-thirds full. Sprinkle a teaspoon of granulated sugar on top of each muffin.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Cool muffins in the pan for 5 minutes before transferring to a wire rack to cool completely.