01 - Set oven to 350°F (175°C). Lightly grease a nonstick donut pan.
02 - In a large bowl, whisk mashed bananas, milk, eggs, vegetable oil, granulated sugar, and vanilla extract until the mixture is smooth.
03 - In a separate bowl, sift together all-purpose flour, baking powder, baking soda, ground cinnamon, and salt until evenly combined.
04 - Fold the dry ingredients into the banana mixture gently until just combined; avoid overmixing to maintain a tender crumb.
05 - Transfer batter to a piping bag or a resealable plastic bag with a snipped corner. Pipe batter into the prepared donut pan, filling each cavity about three-quarters full.
06 - Bake in the preheated oven for 12 to 15 minutes until golden and the tops spring back when lightly pressed.
07 - Remove pan from oven and allow donuts to rest in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely.
08 - Whisk powdered sugar, 2 to 3 tablespoons of milk, and vanilla extract together until smooth. Dip cooled donuts into the glaze and let set for a few minutes before serving.