Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, vegetable oil, eggs, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Fold in the chocolate chips if using.
Divide the batter evenly among the muffin cups, filling each about ⅔ full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.