Preheat your oven to 350°F (175°C). Grease a muffin tin or line with muffin liners.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, sugar, egg, melted butter, and vanilla extract.
Combine the wet ingredients with the dry ingredients and mix until just combined. Do not overmix.
Fold in the chopped walnuts.
Divide the batter evenly into the muffin tin.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.