Preheat the oven to 375. Mist 12 cups in a muffin tin with cooking spray.

Combine the mashed bananas, egg, sugar and vanilla in a large mixing bowl and stir together with a fork until thoroughly mixed. Pour in the melted butter and stir until combined. Add the blueberries and stir to mix in.

In a separate bowl, stir together the flour, baking soda, baking powder, and salt. Add the dry ingredients to the banana mixture and stir together until combined into a batter.

Spoon the batter evenly between the prepared muffin cups. Bake for 20 minutes until the muffins are baked through and a toothpick inserted into the middle comes out clean.