GreaseNever use margarine, shortening or, for that matter, grease. an 8-cup soufflé dish or 4-2 cup oven safe ramekins with the cold butter. Add the grated Parmesan and roll around the dish to cover the sides. Cover with plastic wrap and place into the freezer for 5 minutes.
Heat oven to 375ºF.
Melt the butter in a 3-quart saucier over medium heat. Simmer until all the water has cooked out of the butter and it stops bubbling, about 2 minutes. Combine the flour, mustard, garlic powder, and salt in a small bowl. Whisk this mixture into the melted butter. Cook for 2 minutes, or until the roux appears dry.
Whisk in the hot milk and turn the heat to high. As soon as the mixture reaches a boil (this should happen immediately), remove from the heat.
Beat the egg yolks in a medium mixing bowl until creamy, about 1 minute. Temper the eggs by slowly adding a few ounces of the milk mixture to the yolks, whisking constantly. Add another few ounces of the milk mixture to the yolks, and then pour the yolk mixture back into the saucier, whisking vigorously. Remove the pan from the heat and whisk in the cheese. Transfer this mixture back to the medium mixing bowl and set aside. At this point, the mixture can be covered with plastic wrap and refrigerated for up to 1 week. Bring it to room temperature before using.
Using a hand mixer, whip the egg whites, water, and cream of tartar in a clean metal bowl until glossy and firm.
Time to fold. Start by quickly stirring a quarter of the egg white foam into the base. Using a spatula, fold in the remaining foam in thirds. The spatula goes down right in the middle, then sweeps up the side and turns its load over. Turn the bowl a quarter and repeat the exact same action.
Immediately pour the mixture into the chilled soufflé dish, leaving 1/2 inch between the batter and the top of the dish. Place on an aluminum pie pan and bake in the oven for 30 to 35 minutes. Do not open the oven door during the first 30 minutes of baking. When done, a paring knife inserted in the middle of the soufflé should come out mostly clean. The internal temperature should be between 180 and 185°F. Serve immediately.