Wash the kabocha pumpkin thoroughly and remove the skin and seeds.
Place the kabocha pumpkin in a steamer and steam until tender.
Mash the cooked kabocha pumpkin and let it cool.
In a bowl, combine the mashed kabocha pumpkin, flour, sugar, salt, yeast, milk, egg, and butter, then knead into dough.
Cover the dough with plastic wrap and let it rise in a warm place for the first proofing.
Divide the dough into equal pieces and shape into balls.
Place them on a baking pan for the second proofing.
Bake in a preheated oven at 180°C (356°F) for 20–25 minutes.
Cool the bread on a wire rack.