01 - Preheat oven to 350°F (175°C). 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 fully combined and smooth.
03 - In a separate bowl, sift together all-purpose flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Gently fold the sifted dry ingredients into the wet mixture until just combined. Avoid overmixing the batter.
05 - Transfer batter to a piping bag or resealable plastic bag with a snipped corner. Pipe the batter into each donut mold, filling about three-quarters full.
06 - Bake in the preheated oven for 12 to 15 minutes, until donuts are golden and the surface springs back when lightly pressed.
07 - Remove tray from the oven and allow donuts to cool in the pan for 5 minutes. Carefully transfer to a wire rack to cool completely.
08 - For the optional glaze, whisk together powdered sugar, milk, and vanilla extract until smooth. Dip cooled donuts into the glaze, allowing the excess to drip away. Let set for several minutes before serving.