Slice the tofu lengthwise into two slabs, place in pot, cover with water, and bring to a boil. Lower heat and simmer for 3 minutes.
Drain, then press the tofu for 30 minutes.
Crumble the pressed tofu into the food processor, then add the smaller amounts* of the remaining ingredients. Process for several minutes until perfectly smooth, stopping and scraping down the sides as needed.
Taste and add more umeboshi (for saltiness and tang) and/or lemon (for sourness).
You should end up with a thick, rich cream.