01 - Preheat oven to 350°F. Lightly grease a standard 12-cavity donut pan with oil or nonstick spray.
02 - In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, ground cinnamon, and salt until evenly distributed.
03 - In a separate bowl, combine the mashed bananas, eggs, vegetable oil, whole milk, and vanilla extract. Whisk until smooth and no large banana lumps remain.
04 - Pour the wet mixture into the dry ingredients and fold gently with a spatula until just combined. Stop mixing as soon as no dry flour is visible to keep the donuts tender.
05 - Spoon the batter into the prepared donut pan cavities, filling each one about three-quarters full. Alternatively, transfer the batter to a piping bag for cleaner filling.
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 invert onto a wire rack to cool completely before glazing.
08 - Whisk together the powdered sugar, milk, and ground cinnamon until smooth. Drizzle over the cooled donuts and let the glaze set before serving.