01 - Preheat oven to 350°F. Lightly grease a standard donut pan with butter or nonstick cooking spray.
02 - In a large mixing bowl, mash the ripe bananas until smooth. Add milk, melted butter, eggs, and vanilla extract, then whisk until fully incorporated.
03 - In a separate bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and ground cinnamon until evenly distributed.
04 - Pour the dry ingredient mixture into the wet ingredients and fold gently with a spatula just until combined. Avoid overmixing to keep the donuts tender.
05 - Transfer the batter into a piping bag or a large 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 rest in the pan for 5 minutes, then carefully transfer them to a wire cooling rack.
08 - Once the donuts have cooled slightly, combine powdered sugar with ground cinnamon and dust evenly over the tops.