Place cauliflower florets and garlic in a steamer basket over boiling water, cover and steam until very tender, 12-15 minutes. (Alternatively, place florets and garlic in a microwave-safe bowl with 1/4 cup water, cover and microwave on high for 3-5 minutes.)
Place the cooked cauliflower and garlic in a food processor. Add buttermilk, 2 teaspoons oil, butter, salt and pepper; pulse several times, then process until smooth and creamy.
Transfer to a serving bowl. Drizzle with the remaining 2 teaspoons oil and garnish with chives, if desired. Serve hot.