Preheat the oven to 190 degrees C and put a baking tray in the oven to heat. Lightly grease a large souffle dish (about 1 3/4 litres) with the butter. Mix together the Parmesan and breadcrumbs then sprinkle half of this mixture over the bottom and side of the dish, turning the dish to coat evenly. Set aside.
Cut the cauliflower into small florets and cook in a steamer over boiling water for 8–10 minutes until very tender. Tip onto a plate and leave to cool for a few minutes then puree with 100ml of the milk in a food processor or blender to a smooth consistency.
Mix the cornflour with a little of the remaining milk to make a smooth paste. Heat the rest of the milk until almost boiling then pour onto the cornflour mixture, stirring constantly. Return to the pan, add the bay leaf and stir over a medium heat until the sauce is thickened and smooth.
Pour the sauce into a large bowl and remove the bay leaf. Beat in the egg yolks, one at a time. Add the Stilton and stir until melted. Stir in the cauliflower puree, mustard and chives. Season to taste.
Whisk the egg whites in a large, clean bowl, until stiff peaks form. Whisk one-third of the whites into the sauce mixture to lighten it then gently fold in the rest of the whites.
Spoon the mixture into the prepared souffle dish and sprinkle the top with the remaining breadcrumb mixture. Set the dish on the hot baking tray and bake for 35 minutes or until well risen and golden brown. Serve at once.