Boil the sweet potatoes until tender. Remove from heat and peel the skin then mash.
Add the flours a little at a time to the mashed potatoes and knead together to form a firm dough. Add a little water if it's too dry.
Roll the dough on a floured flat surface into a long roll about 5 cm in diameter. Cut the roll into 12 pieces. Shape each piece into a round doughnut with a hole in the centre.
Heat oil in wok until hot and deep fry the doughnuts. Drain on paper towel and set aside.
Put sugar and water in a saucepan to make a thick syrup. Add doughnuts to the saucepan to glaze them completely with the syrup, remove and let cool.