01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or lightly grease each cup.
02 - In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and ground cinnamon until evenly distributed.
03 - In a large bowl, whisk the mashed bananas, sugar, vegetable oil or melted butter, eggs, milk, and vanilla extract until smooth and well combined.
04 - Pour the dry ingredient mixture into the wet ingredients and fold gently until just combined. Be careful not to overmix, as this can make the muffins tough.
05 - Gently fold the blueberries into the batter using a spatula, distributing them evenly without crushing.
06 - Divide the batter evenly among the prepared muffin cups, filling each one about three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean and the tops are golden.
08 - Let the muffins cool in the tin for 5 minutes, then transfer them to a wire rack to cool completely before serving.