Steam cauliflower until cooked and tender, then place steamer with cauliflower lid off, to let cauliflower dry out, drain and cool slightly. Use a tea towel or strainer to squeeze excess water out of the cauliflower.
Place cauliflower in food processor and process until smooth and fluffy. Add butter, sour cream and garlic powder and process until the texture resembles that of mashed potatoes.
Transfer cauliflower mash into a bowl, add 3/4 of the cheese and 3/4 of the chives and stir through until well combined.