Prepare pork belly. Pat skin with paper towels until completely dry. Mix together marinade including soy sauce, sake, brown sugar, mirin, and garlic. Place pork belly flesh only in the marinade, leaving the skin dry. Marinate for at least one hour or overnight.
Preheat oven to 375 degrees F. Fill a roasting pan with 1 inch of water. Place a wire rack on the pan and lay the pork belly on top, skin side up. Spread the salt in an even layer on the skin. Roast for 40 minutes. Remove pork from the oven and increase the oven to 450 degrees F. Remove the salt crust from the pork and continue roasting for another 30 minutes or until the skin is crisp. Remove from heat and let cool slightly. Cut into bite-size pieces and set aside.
Prepare the tare. Combine all of the ingredients for the tare in a medium saucepot. Bring to a boil then reduce heat to low and simmer for 10 minutes. Strain and set aside.
Bring a medium pot of water to a roaring boil. Cook the noodles for 1 minute. Drain immediately.
Add 1 tablespoon shoyu tare and 2 tablespoon pork fat into each bowl. Portion the noodles into the 4 bowls. Top with pork belly, egg, nori and green onions. Serve, instructing everyone to first mix their noodles. Drizzle with the desired amount of vinegar and chili oil and mix again. Enjoy.