1. Preheat the oven to 350°F and grease a standard doughnut pan well.

2. Mash the ripe banana in a large bowl until mostly smooth.

3. In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.

4. Add the brown sugar, granulated sugar, egg, Greek yogurt, milk, melted butter, and vanilla to the mashed banana, then whisk until combined.

5. Fold the dry ingredients into the wet mixture just until no flour streaks remain.

6. Pipe or spoon the batter into the doughnut pan, filling each well about two-thirds full.

7. Bake for 10 to 12 minutes, or until the doughnuts spring back lightly and a toothpick comes out clean.

8. Cool the doughnuts in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

9. Warm the heavy cream until steaming, pour it over the chopped chocolate, and let it sit for 2 minutes.

10. Stir the chocolate mixture until smooth, then whisk in powdered sugar until glossy and thick enough to dip.

11. Dip each cooled doughnut into the chocolate glaze and sprinkle with chopped nuts or toffee bits while the glaze is still soft.

12. Let the glaze set for a few minutes before serving.