Preheat the oven to 350°F (175°C) and grease a muffin tin or line it with muffin liners.

In a mixing bowl, whisk together the flour, baking soda, salt, and cocoa powder.

In another bowl, combine the mashed bananas, granulated sugar, brown sugar, vegetable oil, eggs, and vanilla extract until well mixed.

Gradually add the dry ingredients to the wet ingredients, stirring until just combined.

Fold in the dark chocolate chips.

Spoon the batter into the prepared muffin tin, filling each cup about 2/3 full.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.

Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.