Cut yam into finger-thick slices or cubes. Steam over medium heat for 20 to 25 minutes, until soft and a chopstick can poke through the yam easily.
500 grams peeled yam
Transfer yam into a mixing bowl and mash while it's still hot. Add in Part 1 water and mix until smooth, adding more water if the paste is very thick. Do note that the more water you add, the easier it will be to strain it later, but conversely it will take a longer time to cook.
150 grams water
Strain the yam paste into a non-stick pan. Add in sugar and oil. Dissolve wheat starch into Part 2 water and add the mixture into the pan as well.