Cook the Pasta: Bring a large pot of salted water to a boil. Add the pasta and cook until al dente according to the package directions. Drain and set aside.
Make the Roux: In a large saucepan, melt the butter over medium heat. Once melted, add the flour and whisk continuously for about 2-3 minutes, until the mixture is bubbly and golden.
Add Milk and Cream: Gradually whisk in the milk and cream, ensuring there are no lumps. Continue to whisk for about 5-7 minutes, or until the sauce thickens.
Add Cheese: Reduce the heat to low and add the shredded white cheddar and American cheese. Stir until the cheese is completely melted and the sauce is smooth.
Combine Pasta and Sauce: Add the cooked pasta to the cheese sauce, stirring until well coated. Season with salt, black pepper, and ground mustard (if using).
Serve: Serve immediately, and enjoy your homemade version of Panera Bread’s mac and cheese!