Place cauliflower in a medium sized saucepan and cover in water. Bring to a simmer and cook until tender, about 10-15 minutes.
Meanwhile, heat butter in a large saucepan until it starts to bubble.
Stir in flour and cook, while stirring, for one minute.
Remove pan from heat and whisk in the milk.
Return pan to heat and stir until the sauce thickens. It will be quite thick at this stage. Remove pan from heat.
When the cauliflower is tender, drain and purée.
Stir cauliflower purée, mature cheddar, mustard and cayenne into the sauce.
Season with salt and pepper, to taste, and serve sauce with pasta.
This recipe makes enough sauce to feed four adults, however, if you are just feeding small children with it leftovers freeze very well. Freeze in individual child-sized portions for a quick mid-week meal.