01 - Preheat oven to 350°F. Lightly grease a 12-cavity donut pan with cooking spray or butter.
02 - In a large bowl, mash bananas until smooth. Whisk in melted butter and brown sugar until combined.
03 - Add eggs, milk, and vanilla to banana mixture. Whisk until smooth and incorporated.
04 - In separate bowl, whisk together flour, baking powder, baking soda, cinnamon, nutmeg, and salt.
05 - Gradually fold dry ingredients into wet mixture until just combined. Avoid overmixing to keep donuts tender.
06 - Spoon or pipe batter evenly into prepared pan, filling each cavity about 3/4 full.
07 - Bake for 10-12 minutes until toothpick inserted comes out clean and tops are golden.
08 - Let cool in pan for 5 minutes, then transfer to wire rack to cool completely.
09 - Whisk powdered sugar, 1-2 tablespoons milk, and vanilla until smooth. Drizzle over cooled donuts.