Preheat your oven to 180C / 350F / Gas mark 4 and lightly grease you doughnut pan.
In a medium sized mixing bowl add your flour, sugar, baking powder, cinnamon and dark chocolate chips and mix until everything is well combined.
In a microwavable bowl or jug melt your butter for about 30 seconds in the microwave. Mix in the yogurt, egg milk and vanilla extract. Whisk until everything is well combined.
Stir the banana into the liquid ingredients then pour the wet into the dry ingredients and gently mix to combine everything.
Pipe or spoon your mixture evenly into the 6 different doughnut holes and pop the pan into your oven for 10 to 12 minutes. You are wanting your doughnuts to be firm to the touch and a lovely light golden colour.
Once baked take them out of the oven and leave them in the pan for a few more minutes then take them out and allow then to fully cool down on a wire rack.
Once fully cool spoon a teaspoon on Nutella onto each doughnut and spread to cover the top. More or less can be added according to taste.
Kept in an airtight tin these doughnuts will be good for 3 days.