01 - Preheat oven to 350°F (175°C). Lightly grease a nonstick donut pan to prevent sticking.
02 - In a large mixing bowl, whisk mashed bananas with milk, eggs, vegetable oil, granulated sugar, and vanilla extract until the mixture is smooth.
03 - Sift all-purpose flour, baking powder, baking soda, ground cinnamon, and salt into a separate bowl and mix well.
04 - Gently fold the dry ingredient mixture into the wet mixture, stirring only until just combined; avoid overmixing to maintain a light texture.
05 - Transfer batter into a piping bag or resealable plastic bag with a snipped corner and pipe into prepared donut pans, filling each cavity about three-quarters full.
06 - Bake for 12 to 15 minutes or until donuts are golden and tops spring back when lightly pressed.
07 - Remove from oven and allow donuts to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
08 - Whisk powdered sugar, 2 to 3 tablespoons milk, and vanilla extract until smooth. Dip cooled donuts into glaze and allow to set for several minutes before serving.