In large bowl, combine flour, sugar, baking powder, cinnamon and salt. Make a well in the center of dry ingredients and add the buttermilk, banana, egg, vanilla and melted butter. Stir to combine ingredients.
Spoon batter evenly into lightly greased donut pan. Bake 10-12 minutes. Donuts should spring back when lightly touched. Remove from oven, let cool.
Make the glaze:
In small bowl, whisk together the powder sugar, milk and vanilla until smooth and creamy. Dip each cooled donut in the glaze. Sprinkle with cinnamon.