Preheat an oven to 400 degrees F (200 degrees C). Line a large baking dish with aluminum foil.
Generously spread the softened butter over the flesh of the squash; lie in the prepared baking dish with the cut sides facing down.
Bake in the preheated oven for 30 minutes. Flip the squash and put about half the butter and about half the brown sugar in each of the cavities of the squash halves. Return to the oven until the outer shell is soft, about 10 minutes more. Spoon the flesh of the squash into a bowl to serve.