Place the cauliflower florets in a steamer basket in a pot with water. Cover the pot, bring the water to a boil, and steam the cauliflower for 10-15 minutes until very tender.
Drain the cauliflower and pat it dry. You want to remove as much excess moisture as possible so the finished dish isn't watery.
Transfer the steamed cauliflower to a food processor. Add milk, Greek yogurt, butter, salt, and pepper to the bowl. Cover and process until smooth, scraping the sides as needed.
Taste the cauliflower and add salt, pepper, or more butter as desired. Serve warm with a little extra butter and chopped chives on top.
Notes
Feel free to add any of your favorite potato toppings to this mashed cauliflower: extra butter, chives, a sprinkling of paprika, shredded cheese or cooked, crumbled bacon.