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 the mixture is completely smooth.
03 - In a separate bowl, sift together all-purpose flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Gently fold the dry ingredients into the wet mixture, mixing only until just combined. Avoid overmixing to preserve a tender crumb.
05 - Spoon the batter into a piping bag or resealable plastic bag with the corner snipped. Pipe batter into the prepared donut pan, filling each mold about three-quarters full.
06 - Bake the donuts for 12 to 15 minutes, or until golden and the tops spring back when lightly pressed.
07 - Remove the pan from the oven and allow donuts to cool in the pan for 5 minutes. Transfer to a wire rack and cool completely.
08 - For the glaze, whisk powdered sugar, 2 to 3 tablespoons milk, and vanilla extract in a bowl until smooth. Dip cooled donuts into the glaze and let set for several minutes before serving.