Place the soybeans in a large bowl and cover with water by at least 2 inches. Soak until the beans split apart easily when pressed and are no longer chalky in the center, about 12 to 24 hours.

Drain the beans and transfer them to a blender. Add the blending water and blend on high speed until the mixture is completely smooth and milky.

Pour the soybean mixture into a large pot. Bring to a boil over medium-high heat—stay close, as the foam can rise and spill over quickly—then reduce heat to medium-low and simmer until the liquid has thickened, about 15 minutes.

Line a fine-mesh strainer with several layers of cheesecloth and set it over a large bowl. Pour the mixture into the cloth, then gather the corners and squeeze firmly with gloved hands to extract as much soy milk as possible.

Rinse the pot to remove any residue and return the soy milk to it. Bring the milk back to a boil over medium heat, then immediately remove the pot from the heat. Stir in the first portion of nigari and let the mixture rest for 2 minutes.

Gently stir the milk again and add the remaining nigari. Let the pot sit undisturbed for 5 minutes; you should see large white curds separating from a clear, yellowish liquid.

Line a tofu mold with cheesecloth and place it over a tray to catch the liquid. Ladle the curds into the mold, fold the cloth over the top, and apply the weight. Press until the tofu feels firm and holds its shape, about 20 minutes.

Carefully remove the tofu from the mold and peel away the cheesecloth. Serve immediately while warm with a splash of soy sauce, or submerge in cold water for storage.