Preheat oven on 180c. Butter 6 x 1 cup oven proof cups or soufflé dishes and place on a tray. Melt the butter in a saucepan, add the flour and cook, stirring for 2 minutes. Add the milk all at once, whisking continuously until thickened. Cool slightly, add mustard, and then beat in yolks one at a time. Stir in cheese and season to taste.