Place the pork belly in a Dutch oven or large skillet with the soy sauce, mirin, water, and brown sugar. Bring to a simmer, cover, and cook for 60 to 75 minutes, turning occasionally.
Transfer the pork to a board and let it rest. Strain and reserve the cooking liquid for serving if desired.
Boil the eggs for 6 1/2 minutes, then transfer them to ice water. Peel and set aside.
Make the Broth:
Bring the chicken stock, soy sauce, mirin, rice vinegar, sesame oil, garlic, and ginger to a gentle simmer.
Simmer for 15 minutes, then strain out the garlic and ginger. Keep the broth hot.
Cook and Assemble:
Cook the ramen noodles according to their package directions. Drain thoroughly.
Slice the chashu and soft-boiled eggs. Divide the noodles among four bowls and pour in the hot broth.
Top with chashu, egg, bamboo shoots, scallions, and nori. Serve immediately.
Notes
Keep the broth at a gentle simmer so it stays clear and balanced.
Cook noodles separately to prevent a cloudy broth and over-soft noodles.
Chicken thighs can replace pork belly for a lighter variation.