Place a metal steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Cover, bring the water to a boil, and add the yam cubes. Cover, and steam until very tender, about 30 minutes.
Place the steamed yam cubes into a bowl, and mash them with the sugar and vegetable oil until very smooth.
Clean the steamer insert, and line with parchment paper. Place the mashed yam pudding into the steamer over a saucepan of water as before; bring to a boil, cover, and steam for 30 minutes. Serve hot.