01 - Preheat oven to 350°F. Lightly grease a standard donut pan with oil or non-stick spray.
02 - In a large 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, stirring until smooth and well incorporated.
04 - Pour the wet mixture into the dry ingredients and fold gently until just combined. Be careful not to overmix, as this can make the donuts tough.
05 - Spoon or pipe the batter into the prepared donut pan, filling each cavity approximately 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 carefully transfer them to a wire rack to cool completely.
08 - If desired, whisk together the powdered sugar, milk, and cinnamon until smooth, then drizzle over the cooled donuts.