Boil the sweet potatoes until slightly soft, and make a puree when it is still warm.
Mix some of the sweetener to make it only a little sweeter.
Mix the grounded black sesame seed with enough liquid sticky sweetener to make a quite sweet paste (not too sweet, but a little more than it should be if you were going to eat it like that, like a paste).
With the potatoes puree make tennis size balls and introduce one finger to make a hole on one side.
Make the hole big enough to introduce some of the sesame paste. Close the ball with the potato puree.
Cover the balls with the white sesame.

Sweet potatoes
Grounded black sesame seeds
Whole white sesame seeds
Agave syrup or maple syrup (no cane sugar, it needs no be sticky)