​Melt the butter in a small pan, add the flour and mustard powder. Mix to a smooth paste then cook, stirring, for 30 seconds. Gradually add the milk stirring well between additions, then slowly bring to the boil, stirring until the sauce thickens. Add half the cheese. Remove from the heat, season and cool.
Preheat oven to 180C/Fan 160C/ Gas Mark 4. Grease and base line four 250ml/8floz ramekins. Place in a roasting tin. Put the kettle on to boil.
Stir the egg yolks into the cooled sauce. Whisk the egg whites in a clean grease-free bowl until they form stiff but not dry peaks. Using a large spoon lightly fold in a heaped spoonful, then gently fold in the remainder.
Spoon into the prepared ramekins filling 3/4s full. Pour enough boiling water from the kettle to come half way up the outside of the ramekins. Bake for 20mins until risen and golden. Cool.
To serve: Preheat oven to 200F/400F/Fan 180/Gas 6. Run a knife round each souffle and turn out onto a baking sheet. Sprinkle over the remaining cheese. Bake for 10mins until golden and risen. Serve warm with salad.