Cut the kabocha in half with a sharp knife and scoop out the seeds. Trim off the skin, then cut into small wedges (about 1 inch thick x 3 inches long).
Wash, peel and cut the potatoes into 1 inch cubes. Place the potatoes into a large pot, then add the kabocha on top of the potatoes. Fill the pot with water making sure to cover the top of the kabocha; season generously with salt. Bring the water to a boil. Reduce heat to medium-low; cover and boil gently for 15 to 20 minutes or until potatoes and kabocha break apart easily when pierced with a fork. Drain well and return the cooked potatoes and kabocha to the same pot, then add the cream and butter.
Mash to desired consistency with potato masher or whisk. Season to taste with salt and pepper. Serve immediately alongside your favorite meat, poultry or fish entrée and green vegetable for a satisfying seasonal meal!