Combine the stock and water in a pot with 2 teaspoons of salt. Bring to a boil, then add the macaroni. Cook the macaroni just under what is recommended on the package as the macaroni will continue to cook in the oven. Reserve 2 tablespoons of the stock, drain the macaroni and return to the pot. Add the reserved stock, and the oil, season with salt and pepper, toss to mix and set aside.

Preheat the oven to 425 F and butter a 2 quart baking dish. Coat the inside of the dish with 1/2 cup of the Parmesan cheese, turning and tipping to ensure equal distribution of the cheese. This makes a nice crunchy crust.

You are now going to make a cream sauce. Melt the butter in a saucepan over medium heat. Add the flour and cook to make a roux. Whisk in the milk and cook, stirring often until thickened, which will be about 5 minutes. Stir in the mustard powder. Remove from the heat and let cool slightly. Add the egg yolks, one at a time, beating well after each addition. Add the two types of cheese and stir until melted. Season with salt and pepper and let cool.

Whisk the egg whites with a pinch of salt until stiff peaks form. Gently fold the beaten whites into the cooled cheese sauce. Gently fold half of the mixture into the macaroni and add to the prepared dish. Gently spoon the rest of the mixture on top.

Bake until puffed and browned, about 25 minutes. Serve within 5 minutes while the souffle top is still puffed.