Cut the cauliflower into small florets and steam, either on the stove or in the microwave. It should fall apart when you poke it with a fork if it's cooked enough.
Drain the cauliflower well
Blend the cauliflower in a blender with the cubed butter, parmesan, cream, (optional) garlic, salt and pepper. Depending on the strength of your blender/food processor this could take 30 seconds - 2 minutes. (If you don't have a blender, you can manually mash it with a potato masher.)
Top with fresh parsley if you have it. Don't sweat it if you don't
Notes
Note if it doesn't blend smoothly, the cauliflower should have been cooked longer. When in doubt, cook it more not less! You can't really overcook it, so keep cooking until a fork goes through the cauliflower like butter.