Preheat the oven to 350° F (176°C).
Line a baking sheet with parchment paper or a silicon mat.
Halve the squash and remove seeds by scraping with a spoon.
Drizzle olive oil on each half and spread it around so it coats the flesh of the squash.
Sprinkle it with salt and pepper.
Place squash halves, cut side down, on the baking sheet.
Roast in the oven for about 45 minutes, until squash is tender and can easily be pierced with a fork. The timing may vary depending on the size of your squash.
Remove from the oven and cool for 5 minutes.
Scoop the flesh out of the squash halves and put it in a bowl. Add the butter.
Mash well with a potato masher or hand mixer.
Season again with salt and pepper, to taste. Garnish with parsley and serve.