If you have a microwave, poke kumara with a fork and microwave 6-8 minutes, until soft. Once cooked, peel off skin and mash with a fork. If you don't have a microwave, you will need to peel and chop your kumara and boil it, until tender.
In a large bowl, whisk together milk and egg, add flour, baking powder and kumara. Stir until combined and resembles a thick paste.
Place oil in a small pot, until about 5cm deep. Heat on medium heat. Place a small piece of dough in the oil, once it starts bubbling, the oil is ready. Alternatively, if you have a cooking thermometer, heat oil to 180°C. If you'd prefer to bake your doughnuts instead, preheat oven to 180°C. Grease a mini muffin tin and fill with ¼ cup batter in each cup. Bake for about 20 minutes, until golden and cooked through.
To test your oil temperature, do one doughnut first. Using two teaspoons, spoon a small amount of better onto one spoon, then push it into the oil, away from you, using the other spoon. Be careful, the oil is very hot! Cook doughnut for about 4 minutes, until golden and cooked through. If your doughnut starts to burn before it's cooked in the middle, turn the heat down.
Continue this process with the remaining batter, cooking 3-4 at a time. Transfer to a paper towel once cooked.
Place sugar and cinnamon in a small bowl and toss cooked donuts in it, then transfer to serving dish to enjoy. If you baked your doughnuts, toss them in a little melted butter before tossing in cinnamon sugar.