Preheat the oven to 425 degrees F. Butter a 9-by-13-inch baking pan.
In a large bowl, combine the Cheddar, butter cubes, sugar, ground mustard and 1/2 teaspoon salt.
Bring a large pot of salted water to a boil over high heat. Add the macaroni and cook until al dente, a minute or so less than the recommended cooking time on the package. Drain well. While still hot, dump the macaroni into the Cheddar-butter mixture and toss to mix well.
Meanwhile, in a small saucepan, bring the milk to a bare simmer over medium heat. In a medium bowl, whisk the egg briefly to break it up. Temper the egg by drizzling 1/2 cup of the warm milk in while whisking until incorporated. (This will prevent the egg from scrambling.) Mix the tempered egg and the remaining warm milk into the macaroni and stir well until everything is evenly coated.
Pour the contents of the bowl into the prepared baking pan, spreading the macaroni and sauce evenly.
In a medium bowl, toss together the pancetta, Parmesan, panko and parsley, and sprinkle over the top of the macaroni. (This will create a yummy crust.)
Bake until golden brown and bubbling on top, and the pancetta is crispy, 25 to 30 minutes. Serve warm.