In a pan on medium heat, pan fry pork belly until 80% cooked (timing will depend on thickness of pork)
Using a paper towel, soak up excess oil from pork belly
Add in soy sauce, mirin, sake, sugar
Stir well and simmer until the sauce caramelizes and thickens up. This should only take a few minutes. Keep an eye out because the high sugar content will cause the pork to burn easily.
Once sauce thickens up and pork is cooked through. Pork is ready.