On a sheet tray, spread 1/4 of the florets with the oil, season with salt and bake until caramelized, about 25 minutes. Meanwhile, combine remaining cauliflower, milk and half a teaspoon of salt in a medium saucepan over medium heat. Bring mixture to a simmer, cover, and cook until cauliflower is tender, about 20 to 25 minutes. Strain cauliflower from milk mixture, reserving both. Transfer cauliflower to a blender. Add remaining 1/2 teaspoon of salt and butter to the blender. Add half of the reserved milk liquid. Secure top on blender and puree mixture until smooth. If mixture is too thick, thin by adding some of the remaining liquid. Season, to taste. Serve in a large serving bowl topped with caramelized florets and chives.