Peel the skin off the sweet potato and slice into 2 cm rounds. Soak in water for a while.
Pour the water and potatoes in the sauce pan and cook over medium heat. When the potatoes soften, drain the water from the pan, then turn the heat back on. Shake the pan constantly to evaporate the liquid.
Remove from the heat and mash the potatoes. While it's hot, add the ingredients marked with ★ in listed order and mix well.
Divide into 3 portions and place onto plastic wraps. Gently squeeze the wrapped portions into a ball.
Unwrap and they're done!
If you want to grill them, brush egg yolk on the surface and grill until the surface turns golden brown.