Start by preparing the homemade dashi broth. Heat water in a pot over medium heat. Once it comes to a simmer, add dried kelp and simmer for 10 minutes. Add the bonito flakes then turn off the heat. Set aside to steep for 10 minutes then strain. For a simpler version, combine 4 1/2 teaspoons of dashi powder with 4 1/2 cups of hot water. Mix until the dashi powder has fully dissolved.
In a pot, combine dashi broth, soy sauce, sake, dried chili peppers, and ginger. Simmer for 5 minutes then set aside.
Make a 3 inch slice on the core or head of the napa cabbage. Split the cabbage in half with your hands, instead of cutting through with the knife. Cut each half in half, which leaves you with 4 sections or quarters
Cut off the core from each section. Lay a piece of pork belly in between each layer of cabbage. Repeat for each section.
Once the cabbage and pork belly have been layered, cut each section into 3 to 4 equal pieces, depending on the height of your pot. Repeat for all sections
Arrange the cabbage and pork layers into a large pot, making sure to pack them in tightly since they will shrink in size while cooking. Place the mushrooms in the center of the pot.
Pour the seasoned broth over the cabbage and pork layers then cover with a lid.
Cook over medium heat for 10 minutes, until the pork has fully cooked through.
Serve hot and enjoy with the dipping sauce!