Preheat the oven to 180C/gas 4. Blitz the banana in a blender until smooth, add the Natvia, yogurt, butter, eggs and vanilla and pulse to combine.
Add the dry ingredients (minus the chocolate) and pulse a few times to mix until the mixture is just combined. Stir through the chocolate chips and pour into a greased doughnut pan or doughnut molds, filling about 2/3 full.
Bake for 15-20 minutes, until puffed up, golden and cooked through (a skewer inserted into a doughnut should come out clean).
Mix the Natvia and cinnamon. While still warm, roll the doughnuts in the melted butter and then in the Natvia-cinnamon mix to coat. Allow to cool slightly and serve warm or cold.