Preheat oven to 350 degrees. Spray donut pan with nonstick cooking spray.
In a medium bowl, beat together the bananas, sugar, egg, vanilla, and melted butter until well combined.
In a small bowl, combine the flour, baking powder, salt, and cinnamon.
Stir the dry ingredients into the wet ingredients until just combined.
Spoon the batter into the donut pan and bake for 13 minutes.
Let cool for 5 minutes before removing from the pan.
While the donuts are cooling, whisk together the powdered sugar and milk.
Drizzle glaze over cooled donuts.