01 - Preheat oven to 350°F. Lightly coat a nonstick donut pan with oil or nonstick spray.
02 - In a large bowl, whisk together mashed bananas, milk, eggs, vegetable oil, granulated sugar, and vanilla extract until a smooth mixture forms.
03 - In a separate bowl, sift together all-purpose flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Gently fold dry ingredients into the banana mixture, stirring only until just combined to avoid overmixing.
05 - Transfer batter to a piping bag or resealable plastic bag with a snipped corner and pipe into the prepared pan, filling each cavity about three-quarters full.
06 - Bake for 12 to 15 minutes, until golden and tops spring back when lightly pressed.
07 - Remove from oven and allow donuts to rest in the pan for 5 minutes. Transfer to a wire rack to cool completely.
08 - For the glaze, whisk together powdered sugar, milk, and vanilla until smooth. Dip each cooled donut and let glaze set before serving.