Preheat oven to 425 degrees F.
Grease a 12 cup muffin tin or line with paper liners.
Sift the flour, baking powder, baking soda, salt & cinnamon together in a small bowl.
Mash the bananas, and then mix in the sugar, egg, vanilla, milk and melted butter in a large bowl. Add the nuts (if using) and mix.
Fold the flour mixture into the banana mixture until smooth.
Spoon the batter into the muffin tin, filling each cup 2/3s full.
Bake in the pre-heated oven about 20-25 minutes. The tops should spring back when pressed and a toothpick should come out clean.
Let stand briefly in the muffin tin, and then turn out onto a cooling rack.