Preheat oven to 375°F (190°C). Line a 12-count muffin pan with muffin liners and set aside.
In a large mixing bowl, whisk together the all-purpose flour, baking soda, ground cinnamon, and salt until well combined. Set aside.
In a separate mixing bowl, whisk together the melted butter, brown sugar, egg, vanilla extract, and mashed banana until fully combined.
Add the dry ingredients to the wet ingredients and mix together until just combined, making sure not to over mix the batter.
Evenly distribute the batter between all 12 liners in the muffin pan.
Bake at 375°F (190°C) for 16-20 minutes or until a toothpick inserted into the center of the muffins comes out clean. Remove from the oven and cool for 5 minutes in the pan, then carefully remove the muffins from the pan, and place them on a wire rack to cool completely.