Peel, wash and cut the sweet potato into cubes. Steam cook for 15 mins or until soft through inside.
Take the cooked sweet potato in a mixing bowl. Mash well.
Add kozhukattai flour (use idiyappam flour or rice flour, if not available), salt, oil and mix well.
Add hot boiling water, just a little to make a dough. Knead well smoothly.
Prepare both purple and yellow version same way. I used a pinch of turmeric for yellow one.
Take a large lemon size ball of the dough and knead to smooth it.
Flatten to a thick, palm sized disc. Place the mixture about 1 tbsp in centre.
Gather the sides to cover the stuffing carefully. Seal & roll to smooth ball.
Place it in a mould and shape it. I used my mooncake mould. You can use any modak shape or even jelly moulds to shape.
Steam for 5 - 7 minutes. I used my idli pot and placed the modak in bamboo steamer. You can use idli plates too. Just grease it and place the modaks. Repeat to finish.