01 - Preheat oven to 350°F. Lightly grease a donut pan with butter or nonstick cooking spray.
02 - In a large mixing bowl, mash the bananas until smooth. Add milk, melted butter, eggs, and vanilla extract, then whisk until thoroughly combined.
03 - In a separate bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon until evenly distributed.
04 - Gently fold the dry ingredient mixture into the wet ingredients, stirring just until combined. Avoid overmixing to keep the donuts tender.
05 - Transfer the batter into a piping bag or a zip-top bag with one corner snipped off. Pipe the batter evenly into each donut cavity, filling approximately three-quarters full.
06 - Bake for 14 to 16 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
07 - Allow the donuts to cool in the pan for 5 minutes, then carefully transfer them to a wire rack to cool completely.
08 - Once slightly cooled, dust the donuts with a blend of powdered sugar and ground cinnamon if desired.