Combine cauliflower and water or chicken stock in a medium saucepan, and bring to a boil over high heat. Reduce heat to a simmer, and cook until cauliflower is very tender, about 10 minutes.
Using a slotted spoon, transfer cauliflower to the bowl of a food processor. Process until smooth, adding 1 to 2 tablespoons cooking liquid, 15 to 20 seconds. Add sour cream and butter, and process 5 to 10 seconds more. Season with salt and pepper. Serve hot.
For a garnish, saute a few sliced cauliflower florets in a bit of butter until tender and lightly golden.