01 - Preheat oven to 350°F. Lightly grease a nonstick donut pan.
02 - In a large bowl, whisk together mashed bananas, milk, eggs, vegetable oil, granulated sugar, and vanilla extract until homogeneous.
03 - In a separate bowl, sift flour, baking powder, baking soda, ground cinnamon, and salt together.
04 - Gently fold prepared dry ingredients into the wet mixture just until blended, avoiding overmixing.
05 - Transfer the batter to a piping bag or resealable plastic bag with the corner snipped. Pipe batter into each cavity of the prepared donut pan, filling each about three-quarters full.
06 - Bake for 12 to 15 minutes, or until donuts are golden and tops spring back when lightly pressed.
07 - Allow donuts to rest in the pan for 5 minutes, then transfer onto a wire rack to cool completely.
08 - Whisk together powdered sugar, milk, and vanilla extract until smooth. Dip cooled donuts in glaze and let set briefly before serving.