Preheat oven to 350 degrees (F). Spray your donut mold with non-stick spray and set aside.
In a medium-sized bowl whisk together the flour, light brown sugar, salt, and baking powder.
In a separate bowl mix the mashed bananas, melted butter, and egg. Gently fold the wet ingredients into the dry ingredients. Mix only until just combined. The batter will be thick.
Spoon the batter into the donut pan and place the donut pan on a cookie sheet.
Put the donuts into the pre-heated oven and bake for 17-20 minutes or until they spring back when lightly pressed.
Remove from oven allow and allow to cool in the pan for 10 minutes and then remove and allow to cool fully.