Cut the bamboo shoots in half lengthwise, then into 6 wedge-shaped pieces. Boil in a pot of water for 3 minutes, drain, and pat dry with a paper towel.

Lay a slice of pork belly vertically, place a bamboo shoot piece on top, and roll it up tightly. Repeat to make 12 rolls. Sprinkle with salt and pepper.

Heat salad oil in a frying pan over medium heat. Place the rolls seam-side down and cook, turning occasionally, until browned. Cover and steam over low heat for 3 minutes until the pork is cooked through.

Remove the lid and blot excess oil with a paper towel. Add sake, mirin, and soy sauce, and cook until the sauce thickens and coats the rolls. Turn off the heat and add butter, letting it melt with residual heat.