Pat the pork belly dry and place skin side down on a tray. Rub the sake and rice wine vinegar into the meat. In a small bowl mix together 2 teaspoons of the salt, the sugar and pepper. Evenly rub the spice mixture into the meat.
Flip the pork belly so it is fat-side up, cover it with aluminum foil then refrigerate it for at least 12 hours and up to 24 hours.
Let the pork belly sit at room temperature for about 30 minutes before it goes into the oven. Preheat oven to 300°F. Use a sharp knife to cut a cross-hatch pattern about 1/4" deep into the fatty side of the pork belly.
Place the pork belly on a roasting pan and cook for 2 hours, until the pork is cooked through and tender. Turn the oven up to 500ºF and continue to cook the pork belly until crispy, about 10 to 15 minutes.
Let the pork belly cool to room temperature. Wrap the pork tightly in plastic and refrigerate until thoroughly chilled, at least 2 hours and up to a few days.
When ready to serve, slice the pork into ½” pieces. Heat a medium sauté pan over medium heat and add 3 tablespoons of olive oil. Brown the pork belly on all sides then remove to a paper towel lined plate.
For assembly:
In a small saucepan over medium heat, add the butter and allow to melt. Add the garlic and sauté until fragrant, about 30 seconds. Whisk in the gochujang, sugar and cashew butter until completely combined. Keep warm until ready to use.
Heat a large nonstick skillet over medium heat, add the pork belly and sauté until crispy on all sides, about 6 minutes. Remove to a large bowl, leaving as much oil in the pan as you can and return the pan to heat.
Add the rice into the pan in an even layer and cook, untouched, until the bottom is crispy and golden brown, about 3 minutes. Stir and cook for an additional 3 minutes. Divide the crispy rice among serving bowls and return the pan to heat.
Add the pork back to the pan and heat for an additional minute then add the gochujang sauce and toss to combine.
Divide among the serving bowls. Top each with the cashews, cilantro, green onions, radish and sesame seeds. Serve immediately.