Preheat the oven to 350 degrees F (180 degrees C) and grease a mini metal donut pan with melted butter, oil, or baking spray.
In a large bowl whisk together the mashed banana, melted butter, brown sugar, and granulated sugar.
Mix in the egg and vanilla.
Whisk in the flour, baking powder, baking soda, salt, and cinnamon until fully combined.
Transfer the batter to a piping bag, then snip the end off and pipe the batter into each donut cavity about half way full. Careful not to over-fill! Alternatively, you can spoon the batter into the cavities, but it will be a little messy.
Bake the donuts for about 10 minutes or until a toothpick inserted comes out clean.
Transfer donuts to wire rack to cool completely before dipping in the chocolate glaze.
For the chocolate glaze:
In a small bowl, heat the heavy cream in the microwave for about 30 seconds.
Add in the chocolate and stir until fully combined and smooth.
Notes
Store donuts at room temperature in an air-tight container for up to 3 days.