Drain the tofu and wrap in paper towels to absorb excess water. Puree in a food processor until smooth, then transfer to a bowl and set aside.
Bring a large pot of salted water to a boil. Fill a bowl with ice water. Cook the spinach in the boiling water until wilted, about 2 minutes, then drain and plunge it into the ice water. Drain and squeeze out the excess water.
Toast the sesame seeds in a skillet over medium-low heat, tossing, until golden, about 3 minutes. Cool, then grind in a spice grinder. Stir the ground seeds, sugar, mirin, soy sauce and spinach into the tofu. Season with salt.
Photograph by Yunhee Kim
Recipe courtesy of Masaharu Morimoto for Food Network Magazine