Preheat oven to 350°F (175°C). Line a muffin tin with paper liners or grease with non-stick spray.
In a large bowl, whisk together flour, cocoa powder, baking soda, and salt.
In another bowl, mash bananas and mix in granulated sugar, brown sugar, oil, egg, and vanilla extract until smooth.
Gradually add the dry ingredients into the wet ingredients and mix until just combined. Do not overmix.
Fold in chocolate chips.
Divide the batter evenly among the muffin cups, filling each about 3/4 full. Top with extra chocolate chips if desired.
Bake for 17–20 minutes or until a toothpick inserted in the center comes out clean.
Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.