Bring the mixture back to a simmer and cook over low heat for 5 to 10 minutes, stirring with a spatula to avoid scorching the bottom of the pan. Strain again through a fine-mesh strainer. Allow to fully cool.
To make the tofu, pour the soymilk mixture into small cups or containers until the mixture is about 1 inch deep. Mix the nigari in the individual cups of soymilk, using a ratio of about 1 teaspoon of nigari to 1 1/2 cups of the soymilk mixture (the ratio is about 2 percent nigari). Sometimes small grains will separate at this point; if so, strain a third time.
Set up a steamer and place the cups of soymilk and nigari into the steamer over low heat, looking for a steady jet of steam rather than puffs, and place the lid ajar to modulate the heat. Steam until the tofu has fully coagulated, checking with a cake tester, 6 to 10 minutes. Let cool.