Using a sharp knife, cut the cauliflower into evenly-sized florets.
Bring water to a boil in a medium-sized saucepan, then add the cauliflower florets and boil them until they're soft. This should take roughly 10 minutes.
Strain the cauliflower through a colander and add it to a food processor with the butter, cream cheese, wholegrain mustard, and salt and pepper. Blend everything until smooth, then serve the cauliflower mash topped with the chopped fresh chives and a little more butter, if desired.