Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners.
In a large mixing bowl, mash the ripe bananas until smooth.
Add melted butter to the bananas and mix well.
Stir in sugar, egg, and vanilla extract until combined.
In a separate bowl, whisk together flour, baking soda, salt, and cinnamon if using.
Gradually add the dry ingredients to the banana mixture, stirring just until combined.
Gently fold in the blueberries.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake for 20-25 minutes, or until a toothpick inserted comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer to a wire rack.