Sauté leek with 1 Tbsp of butter until tender; set aside.
Boil cauliflower until tender (about 6 minutes). Drain cauliflower well; pat dry between several layers of paper towels or clean dish towel to get the moisture out.
Put hot cauliflower, leek, yogurt and 2 Tbsp butter in food processor, blend until smooth.
Transfer to serving dish, top with butter and garnish with chives
Notes
Cauliflower does not freeze well—freezing will preserve its flavor, but tends to break down the texture. Make sure you purchase fresh cauliflower and eat it within 5 days!