Preheat the oven to 400°F. Rub the inside of a soufflé dish with two tablespoons of softened butter. Add the grated parmesan cheese to the dish and shake so that it coats the bottom and sides of the dish. Cut a piece of foil that fits the diameter of the dish. Fold the foil in half and wrap around the dish. Secure the foil with a piece of string. Lightly spray the foil with non-stick spray and then set the dish aside.
In a saucepan, melt the remaining butter over medium heat. Add the flour and whisk together. Slowly add the milk, whisking constantly to prevent lumps. Heat the béchamel sauce until it thickens and bubbles, then turn down the heat. Add the Dijon mustard, salt, paprika, nutmeg, and pepper and whisk to combine. Remove from the heat and whisk in the egg yolks. Transfer to a mixing bowl and set aside.
In a mixer with a whip attachment, beat the egg whites and cream of tartar together until stiff peaks. Begin by folding a quarter of the egg whites into the béchamel. Repeat three more times, gently folding in the remainder of the egg whites. Try not to deflate the mixture by stirring too vigorously.
Gently fold in the grated Gruyère cheese and chopped chives. Scrape the mixture into the prepared soufflé dish. Bake for 10 minutes and then turn the heat down to 375°F. Continue baking for another 20 to 25 minutes until done. Avoid opening the oven door while the soufflé is baking.
The soufflé is done when it is puffed and well browned on top. It should still have a slight wobble. Take the soufflé out and remove the foil collar. Serve immediately and enjoy!