To make the cheese sauce, melt the butter in a small pan. When foaming, add flour and mustard powder, and cook for 1 min, stirring constantly. Take off the heat and add the milk, a little at a time, stirring constantly to get rid of any lumps. Once half the milk has been added, return to the heat and add the remaining milk. Keep stirring and bring to the boil, then turn down to a simmer and cook for 2 mins, until thickened and smooth. Add most of the cheese and some seasoning then take off the heat and add the chives.