01 - Preheat the oven to 350°F. Lightly grease a donut pan with butter or nonstick spray.
02 - In a large bowl, mash the bananas until smooth. Add the milk, melted butter, eggs, and vanilla extract, then whisk until well combined.
03 - In a separate bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon until evenly distributed.
04 - Add the dry ingredients to the wet mixture and fold gently just until combined. Do not overmix to keep the donuts tender.
05 - Transfer the batter to a piping bag or a zip-top bag with one corner snipped off. Pipe the batter evenly into each donut cavity, filling about three-quarters full.
06 - Bake for 14 to 16 minutes, until the donuts are golden brown and a toothpick inserted into the center comes out clean.
07 - Let the donuts rest in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
08 - Once slightly cooled, dust the donuts with a blend of powdered sugar and cinnamon if desired.