Cover and steam cauliflower florets and garlic in a steamer basket over boiling water until very tender, 12 to 15 minutes. (Alternatively, place florets and garlic in a microwave-safe bowl with ¼ cup water, cover and microwave on High for 3 to 5 minutes.)
Place the cooked cauliflower and garlic in a food processor. Add buttermilk, 3 teaspoons olive oil, 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.