Cut Kabocha in half, and remove seeds and fibers from the center. Carefully slice into 1/4" thick pieces, about 1" x 2" in size.
Heat a frying pan at medium low heat, add butter, and put Kabocha slices in a single layer in the pan. Cook 2-3 minutes on one side, turn over, and cook another 2 minutes.
Turn off the heat, add soy sauce, Mirin, and sugar. Coat the Kabocha pieces with the seasonings well. Serve with ground black pepper