01 - Preheat oven to 350°F. Lightly grease a 12-cavity donut pan with cooking spray or butter.
02 - Mash bananas in a large bowl until smooth. Whisk in melted butter and brown sugar until combined.
03 - Beat in eggs one at a time, then add milk and vanilla. Whisk until fully incorporated.
04 - In a separate bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
05 - Gradually fold dry mixture into wet ingredients until just combined. Avoid overmixing to keep donuts tender.
06 - Transfer batter to a piping bag or zip-top bag. Pipe into prepared pan, filling each cavity about 3/4 full.
07 - Bake for 10-12 minutes until golden and a toothpick inserted comes out clean.
08 - Let cool in pan for 5 minutes, then carefully transfer to a wire rack to cool completely.
09 - Whisk powdered sugar with milk and vanilla until smooth. Drizzle over cooled donuts or dip tops.