Place butternut pumpkin, cut sides down, in a baking dish. Pour water into dish around pumpkin halves, about 1cm up the sides.
Bake in the preheated oven until tender and easily pierced with a fork, about 1 1/2 hours. Carefully remove the skin with a fork; it should be very easy to remove. Top pumpkin halves with butter and season with salt and pepper to taste.
To cook a large butternut pumpkin takes 90 minutes. If you have a smaller pumpkin, adjust the baking time.