01 - Preheat your oven to 350°F (175°C). Lightly grease a standard 12-cavity donut pan with oil or non-stick 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 (or melted butter), whole milk, and vanilla extract. Whisk until smooth and fully incorporated.
04 - Pour the wet mixture into the dry ingredients. Gently fold with a spatula until just combined — be careful not to overmix, as this can make the donuts tough.
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 transfer them to 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 allow the glaze to set before serving.