Bring a pot of water to boil, then add in sago, cook under medium heat for 10 minutes. Remove from heat, then let it rest for 15 minutes, sago will become transparent. Rinse it under water, drain well.

Divide the purple yam into 10 pieces. 

In a bowl, mix sugar, water chestnut flour and water together. Add in sago and oil, mix well. 

Brush some oil into a tin, add in half sago, then purple yam filling and the rest of the sago. Steam under high heat for 10 minutes. 

Unmold with toothpicks while it cool.