Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease the cups.
In a large bowl, whisk together flour, baking soda, and salt.
In another bowl, mix the mashed bananas, sugar, melted butter, egg, and vanilla extract until well combined.
Fold the banana mixture into the dry ingredients until just mixed. Gently fold in the blueberries.
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Don't overmix the batter; stir until just combined to keep the muffins light and fluffy.
• Use paper liners or grease the muffin tin well to prevent sticking.
• Fill muffin cups ¾ full to allow space for rising.
• Bake immediately after mixing to get the best rise from the baking powder.
• Let the muffins cool in the pan for a few minutes before transferring to a wire rack to prevent them from becoming soggy.