01 - Preheat oven to 350°F. Grease a standard 6-cavity donut pan with butter or non-stick 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 incorporated.
03 - In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg until evenly distributed.
04 - Gently fold the dry ingredient mixture into the wet ingredients until just combined. Avoid overmixing to keep the donuts tender.
05 - Spoon or pipe the batter into the prepared donut pan, filling each cavity about two-thirds full to allow room for rising.
06 - Bake for 12-15 minutes, or until a toothpick inserted into the center of a donut comes out clean and the tops spring back lightly when touched.
07 - Allow donuts to cool in the pan for 5 minutes, then carefully transfer to a wire cooling rack to cool completely before glazing.
08 - In a small bowl, whisk together powdered sugar, 1 tablespoon of milk, and a pinch of cinnamon until smooth. Add more milk as needed to reach a drizzleable consistency. Drizzle evenly over cooled donuts.