Cook cauliflower florets in salted water: Bring 1 quart of water to a rolling boil in a medium-sized pot. Add 2 teaspoons of salt to the water. Add the cauliflower florets, and cook until cooked through, about 4 minutes. Use a slotted spoon to remove florets to a bowl. Reserve cooking water.
Purée cooked cauliflower: Working in batches, fill a blender half-way with cooked cauliflower, add enough cooking water to the blender bowl so that it comes up a quarter of the way. Purée until smooth. You want the consistency to be that of soup, so if you need to add more water, do so.
Add salt and butter: or each blender batch, add salt and butter to taste (about 1/2 teaspoon of salt, and about a tablespoon or more of butter for each blender batch). Since you are using only lightly salted cooking water for a base, not stock, you will need to salt this more than you would otherwise.
Serve immediately into bowls. Drizzle olive oil over the top and sprinkle with chopped chives.