Pre-heat the oven to 200 C and generously butter 4 x 200ml ramekins, lining the bottoms with a disc of non-stick baking paper.
Melt the butter over a medium heat and stir in the flour, cooking for a minute. Take off the heat and add the milk, little by little, stirring until you have a smooth sauce.
Add the mustard, cheddar and season, cooking for a further 2 minutes, until the cheese melts.
Transfer to a heatproof bowl and leave the cool for 5 minutes, then stir in the egg yolks.
Whisk the egg whites in a bowl until stiff peaks have formed and then fold about ¼ of the whites into the cheese mixture to loosen. Fold in the remaining whites and then divide the mixture between the ramekins.
Place into the oven for 15 - 20 minutes or until the soufflés have risen and are golden, then remove from the oven and leave the soufflés to cool in the ramekins. Release the soufflés from the ramekins once they are cool and place onto a tray. Cover with the cling film and store in the fridge until you are ready for the second bake.
To double bake (on the day): place the soufflés into an ovenproof dish.
Pour double cream over the soufflés and Sprinkle the whole dish thickly with grated parmesan.
Bake at 180C for about 10-15 mins or until the cream has thickened and the soufflé is golden brown. Serve with a green salad and crusty bread to mop up the sauce.