Preheat the oven to 350 degrees F. Grease and flour a donut pan.
Place the melted vegan buttery spread, mashed banana, brown sugar, and vanilla in a bowl and stir well.
Add the gluten free flour, oat flour, baking powder, and salt. Add the water and vinegar and stir until smooth.
Spoon the batter into the prepared pan. Bake at 350 degrees F for about 13-15 minutes. Allow to cool in the pan for 5-10 minutes, then overturn the pan and tap the bottom to release.
Make the icing:
Place the vegan buttery spread in a bowl, and add the powdered sugar and cocoa powder.
Drizzle in some water, and use a mixer to mix on high speed. Add more water if needed to reach the desired consistency.
Frost the cooled donuts. Allow the frosting to sit on the donuts for a bit - it will get that little bit of a hardened coating like a bakery donut.
Notes
Store leftover donuts in an airtight container. They should stay fresh for about 3 days. You could freeze leftover donuts if you like. Store in a freezer safe container for up to 3 months.