01 - Preheat oven to 350°F. Grease a standard 12-cavity donut pan with butter or nonstick spray.
02 - In a large mixing bowl, whisk together the mashed bananas, melted butter, brown sugar, milk, eggs, and vanilla extract until smooth and well blended.
03 - In a separate bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg until evenly distributed.
04 - Gently fold the dry ingredient mixture into the wet mixture using a spatula, stirring until just combined. Do not overmix to keep the donuts tender.
05 - Spoon the batter into a piping bag or use a spoon to fill each donut cavity about two-thirds full.
06 - Bake for 12 to 15 minutes, or until a toothpick inserted into the center of a donut comes out clean.
07 - Let the donuts rest in the pan for 5 minutes, then transfer to a wire cooling rack to cool completely.
08 - If using the glaze, whisk together the powdered sugar, milk, and a pinch of cinnamon until smooth. Drizzle over the cooled donuts and allow to set before serving.