Whisk the yoghurt, milk, peanut butter, honey, cinnamon and vanilla extract together in a medium bowl. Mash the banana well and stir into the mixture. Divide the mixture between 4 ramekins and cover each ramekin with cling film.
Freeze the ramekins for a few hours. Remove from the freezer and sprinkle the chocolate (if using) over each ramekin and serve.