01 - Preheat the oven to 350°F. Lightly grease a nonstick donut pan to prevent sticking.
02 - In a large mixing bowl, whisk together mashed bananas, milk, eggs, vegetable oil, granulated sugar, and vanilla extract until the mixture is smooth and well combined.
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 until just combined, taking care not to overmix.
05 - Spoon batter into a piping bag or a resealable plastic bag with the corner cut off, then pipe into the prepared donut pan, filling each mold approximately three-quarters full.
06 - Bake for 12 to 15 minutes, or until donuts are golden and the tops spring back when lightly pressed.
07 - Remove from oven and let donuts cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
08 - For the glaze, whisk powdered sugar, milk, and vanilla extract until smooth. Dip fully cooled donuts into the glaze and set aside until the glaze is set.