Cut the squash into 2” cubes and add to an appropriately sized pot. Add enough water to cover 80 percent of the squash, add a generous pinch of salt and place a lid on the pot.
Bring to a boil over high heat and then simmer until the squash is tender, adjusting the flame as necessary to keep a low simmer while stirring the squash. When the squash is very tender remove from the flame and set aside with the lid on to cool.
In a small pan heat the olive oil and fry the rosemary until it is crispy, setting it aside on a paper towel and seasoning it with salt.
Toast the pumpkin seeds using the residual rosemary oil and the same hot pan, toss them in a bowl with the rosemary.
When the squash has cooled for 10 minutes, puree until very smooth in a blender in batches, adding the salted butter and a little of the cooking water as you go to reach the consistency of a thick soup. Return the puree to a pot, combining all batches and reheat to serve, checking the seasoning and adjust as necessary.
Ladle into bowl and sprinkle the toasted pumpkin seeds, dried rosemary and sea salt flakes to finish, as well as a drizzle of extra virgin olive oil or pumpkin oil if you have it.