Add along with the filtered water and mix well.
Using a medium flame, whisk the sesame seed mixture frequently. It will continue to thicken. Bring to a boil and stir vigorously, for about 20 minutes; the mixture will change in both texture and color and become more translucent (especially if using white sesame) and will look like a thick pudding.
Let cool to room temperature (this can be done more quickly by placing the mold (in a larger pan and surrounding with ice water) then store in the refrigerator.
Garnish with grated ginger, wasabi, and a drizzle of soy sauce and enjoy!