Combine cauliflower, garlic cloves, and chicken stock or water 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 10-20 minutes.
Using a slotted spoon, transfer cauliflower to a blender or food processor. Process until smooth. (You'll probably want to add a little of the cooking liquid.) Add butter, and process a few seconds more. Season with salt and pepper, and serve hot.