12-Hour Kurobuta Pork Belly

This is a recipe that I've been pulling teeth to get for literally four years. It's from Kai, a modern Japanese restaurant in Greenbelt. They're very secretive about their recipes, so much so that they didn't provide exact proportions for below ingredients. Had to guess. Chef is ex-Nobu in NYC.

baby pork belly (preferably Kurobuta)

5 parts water

2 parts kikkoman soy sauce (low salt)

1 part mirin

1 part sake

2-3 tblsp sugar

dry Japanese mustard

1. place pork belly and remaining ingredients except mustard powder in a pot; bring to boil, then simmer for 12 hours
2. add combo remaining liquids if necessary
3. once finished cooking, remove pork, reduce remaining sauce further
4. mix dry mustard with water
5. serve pork drizzled with reduce sauce, mustard and japanese rice