Wash the outside of the squash thoroughly since the skin will be eaten.
Trim off the stem and blossom ends, then cut each squash into pieces about 2 inches square.
Bring a large pot of water to a boil, add a pinch of salt, then add the squash pieces and reduce heat to medium-low to maintain a gentle simmer.
Simmer until the squash is tender and easily pierced with a fork, about 10–15 minutes; avoid overcooking or it will become mushy.
Drain the cooked squash in a colander and let it drain well for at least 5 minutes to remove excess moisture.
Coarsely mash the drained squash, including the skin, with a potato masher and let it drain another 2–3 minutes.
Transfer the squash to a serving bowl, stir in the butter to melt, and season with salt and freshly ground black pepper to taste; serve hot.