Slice a cauliflower head in half. Remove stem, tough leaves and break into florets.
Fill a large pot with approximately 2 inches of water. Set a steamer basket in the bottom. Place florets in the steamer basket.
Bring to a boil. Cover with a lid and reduce to a simmer, steaming until very tender, approximately 15-20 minutes.
To check tenderness, pierce the stem of a floret with a pairing knife. If it slides in and out easily, it is done.
With tongs, remove cauliflower from the steaming basket. Transfer to a food processor. Add butter. Puree until smooth.
Scrape down the sides of the food processor. Add the sea salt and fresh ground pepper. Puree again to incorporate. Check seasoning and adjust to taste.