steam the yam and sweet potatos until soft and tender. (roughly 6 parts yam to 1 part sweet potato)
make some concentrated sugar solution by dissolving the sugar in heated water
set aside a small portion of steamed yams and put the rest into a blender or food processor. Blend them until smooth, adding the prepared sugar solution to help the blending process.
For the bit of steamed yams that you set aside earlier, coursely mash them before mixing them with the blended contents from the earlier step.
mix in the shallot oil to taste.