Preheat oven to 400˚F. Butter 6 ramekins or individual cast iron pots and coat with the parmesan cheese. Place on a baking tray and set aside.
In a medium sized pot set over medium-high heat, melt the 4 tablespoons of butter. Once melted, add the flour and stir with a wooden spoon until the flour has absorbed all the butter and you have a foamy mixture. Continue cooking, stirring frequently, for another 2 minutes.
To the pot add the milk in a continuous steady stream, whisking constantly to avoid lumps. Cook for another 2 minutes, stirring frequently. Pour into a large bowl and allow to cool slightly.
Separate the eggs between two clean bowls. To the yolks add the salt and mustard and stir well to combine evenly. Using an electric hand mixer, beat the egg whites until you have stiff peaks, adding the cream of tartar half way through.
Stir the egg yolk mixture into the milk mixture. Add the grated cheese. Add a third of the stiff egg whites to this mix and fold gently with a spatula. Continue adding the whites in thirds until all the whites are incorporated. The mixture should appear very fluffy. Ladle into your prepared pans and bake in the preheated oven for 12 minutes. Try not to open the oven before the time is up.
After 12 minutes, the souffles should have risen and be golden all over. Carefully transfer to your serving plates and enjoy immediately. Sadly it does not take long for the soufflés to start falling!