Preheat oven to 350°F (177°C). Spray a donut pan with non-stick spray. Set aside.

Whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg together in a medium bowl. Set aside. Whisk the egg and brown sugar together in a medium bowl until smooth. Whisk in the butter, mashed banana, vanilla, and buttermilk until smooth and combined. Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.

Spoon the batter into the donut cavities—I highly recommend using a large zipped-top bag for ease. Cut a corner off the bottom of the bag and pipe the batter into each donut cup, filling 2 ∕ 3 –3 ∕ 4 of the way full. (See the photo in my chocolate frosted donuts post for a visual.)

Bake for 9–10 minutes or until the edges are lightly browned. Allow to cool for about two minutes and transfer to a wire rack set on a large piece of parchment paper or on a large baking sheet. Bake the remaining donut batter (there is usually enough batter for 2-3 more donuts) and once baked, transfer to the wire rack. Allow donuts to cool down until you can handle them.

Place the chocolate chips, butter, corn syrup, and water in a medium bowl. Melt in 20 second increments in the microwave, stirring after each time, until completely melted and smooth. Dip the tops of the donuts into the chocolate glaze.

Donuts taste best eaten the same day, though they may be covered tightly and stored at room temperature for 2 days.