Drain the cooked cauliflower thoroughly. Place the cauliflower on a clean kitchen towel or several layers of paper towels and press firmly to remove as much excess moisture as possible. This step is key for creaminess.
Transfer the dried cauliflower to a food processor or a large bowl.
Add the butter, heavy cream, Parmesan cheese (if using), salt, and pepper to the cauliflower.
Process or mash until completely smooth and creamy. Scrape down the sides as needed.