Preheat oven to 350 degrees. Spray a donut pan with non-stick spray and set aside.
Mix the banana, flax egg, vanilla, melted butter, and coconut sugar together in a medium bowl.
Add the dry ingredients and mix until just combined.
Spoon the batter into the donut pan, filling to the top.
Bake the donuts for about 17-20 minutes. You’ll know they’re done when you gently press the tops and they feel springy.
Remove donuts from the oven and allow them to cool in the donut pan for about 10 minutes before flipping onto a wire rack. Cool completely before icing.
Icing
Mix the butter, powdered sugar, vanilla extract, and salt together in a small bowl. Add a splash of non-dairy milk and mix until the icing is not too thick.