Steam sweet potato until soft. Peel and mash until smooth.
Combine lukewarm water with the starter dough ingredients in a small mixing bowl. Leave aside to froth for about 10-15 minutes.
Mix sifted flour, milk powder, sugar and baking powder in a large mixing bowl. Add in shortening and mashed sweet potato to mix. Pour in frothy liquid and add the egg. Mix with a wooden spoon until dough comes together.
Place the dough in a lightly oiled bowl and leave to rise for about one hour or until it doubles in bulk.
Punch down the dough and remove onto a floured work tabletop. Roll out the dough slightly thicker than 1cm and stamp out rounds with a doughnut cutter. Gather the snaps and re-roll, then cut out more doughnuts.
Place doughnuts on a non-stick baking paper and cover with a dry tea towel or a lightly greased cling film. Leave to rise for 10-20 minutes or until they feel light and spongy.
Heat oil for deep-frying in a wok over medium heat. Slip a few doughnuts into the oil at a time, leaving enough room for them to expand.
Make sure that the oil is not too hot otherwise the doughnut will not be cooked. If the oil is not hot enough, the doughnut will absorb too much oil.
When the doughnuts are golden brown, turn them over and cook the other side. It takes about 2-3 minutes to cook through.
When done, remove doughnuts from oil and drain briefly on absorbent paper towels before rolling them in cinnamon sugar mixture.
For kids´ stuff: Dip the doughnuts in melted chocolate and sprinkle with sugared coloured rice.