01 - Preheat oven to 350°F. Lightly grease a standard donut pan with oil or nonstick spray.
02 - In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, cinnamon, and salt until evenly distributed.
03 - In a separate bowl, combine the mashed bananas, eggs, vegetable oil, milk, and vanilla extract. Stir until the mixture is smooth and well incorporated.
04 - Pour the wet mixture into the dry ingredients and fold gently until just combined. Avoid overmixing to keep the donuts tender.
05 - Spoon or pipe the batter into the prepared donut pan cavities, filling each about three-quarters full.
06 - Bake for 12 to 15 minutes, or until the donuts spring back when lightly pressed and a toothpick inserted into the center comes out clean.
07 - Let the donuts rest in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
08 - Whisk together the powdered sugar, milk, and cinnamon until smooth. Drizzle the glaze over the fully cooled donuts.