Sift all three flours together. Add the mashed purple sweet potato and knead to combine. Gradually add the thin coconut milk while kneading, alternating between pouring and kneading until all the coconut milk is incorporated and the batter is smooth. Add the salt, granulated sugar, and palm sugar, and knead until everything is fully dissolved and well mixed. Strain the mixture through a fine mesh sieve and set aside. Steam small ceramic cups for 10 minutes to preheat them.

After 10 minutes, pour the pudding base into the preheated cups, filling each about 3/4 full (stir the batter each time before pouring). Cover with a lid (wipe the lid each time before covering to prevent water from dripping onto the pudding). Steam the pudding base for 7 minutes. Then, pour the coconut topping over the base (stir the topping each time before pouring). Steam on high heat. For a slightly oily coconut topping, steam for 10 minutes; for a smooth topping, steam for 8 minutes. Once done, remove the cups and let them cool. They're ready to serve!