Chop the sweet potatoes and soak in water for 15 minutes to remove starch. Remove from the water and pat dry.
Place the sugar, oil, soy sauce and rice vinegar in a pan. Add the dry sweet potatoes. Cover with a lid that you have wrapped in a tea towel (this catches the condensation that builds on the lid). Turn the heat to medium and cook for 2-3 minutes until it starts to bubble. Then reduce the heat and cook with the lid on, turning every 2 minutes, until the potatoes are soft.
Transfer to a bowl and sprinkle with sesame seeds.