01 - Preheat oven to 350°F. Lightly grease a nonstick donut pan.
02 - In a large mixing bowl, whisk together mashed bananas, milk, eggs, vegetable oil, granulated sugar, and vanilla extract until well blended.
03 - In a separate bowl, sift together the all-purpose flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Fold the dry ingredients gently into the wet mixture until just incorporated. Avoid overmixing to maintain a tender texture.
05 - Transfer the batter to a piping bag or a resealable plastic bag with the corner snipped. Pipe the mixture into the prepared donut pan, filling each cavity about three-quarters full.
06 - Bake for 12 to 15 minutes, or until donuts are golden and the tops spring back when touched lightly.
07 - Remove from oven and allow to cool in the pan for 5 minutes. Transfer donuts to a wire rack to cool completely.
08 - For the glaze, whisk powdered sugar with milk and vanilla extract until smooth. Dip each cooled donut in the glaze and allow to set for several minutes before serving.