Rinse off the cauliflower and dry with a paper towel. Place cauliflower on a cutting board and remove any "stems" and leaves. Place both heads of cauliflower in a large pot with enough water to cover an inch above the vegetables. Boil on medium-high heat until cauliflower is tender, about 15 minutes.
Remove cauliflower from pot and allow to drain and cool to the touch. Remove all white "heads" from the cauliflower and place them in a bowl. Discard the stem.
While cauliflower is cooking, rinse and slice the leeks then peel and chop the garlic. In a sauté pan over medium/high heat, melt 3 tablespoons butter. Reduce heat to medium then add sliced leeks and chopped garlic, cooking until leeks are tender, about 10 minutes.
Using a food processor, purée cooked cauliflower and sautéd leeks and garlic in batches. Place purée mixture in a large mixing bowl. Add 2 tablespoons melted butter and stir until well blended. Add the yogurt and blend until purée is creamy. Add salt and pepper and taste for seasonings. Top with chopped chives.
Notes
Helpful Tip: Cauliflower Leek Purée can be made a day in advance and then reheated when ready to serve.