Preheat the oven to 350°F (175°C).
On a baking sheet, toss 1/4 of the cauliflower florets with oil and season with salt.
Roast in the oven until caramelized, approximately 25 minutes.
In a medium saucepan, combine the remaining cauliflower florets, milk, and 1/2 teaspoon of salt over medium heat.
Bring to a simmer, cover, and cook until the cauliflower is tender, about 20 to 25 minutes.
Strain the cauliflower from the milk, reserving both the cauliflower and the liquid.
Place the cauliflower in a blender, adding the remaining 1/2 teaspoon of salt and butter.
Pour in half of the reserved milk.
Secure the blender lid and blend until smooth.
If the purée is too thick, add more of the reserved milk to reach your desired consistency.
Season to taste.
Serve the purée in a large bowl, garnished with the caramelized florets and chopped chives.