Place rack in lower third of oven and preheat to 350˚F (180˚C). Grease four tall soufflé ramekins with butter and coat with the toasted breadcrumbs.
Bring a large pot of salted water to a boil. Cook the macaroni until nearly tender. Be careful not to overcook since the pasta will bake further in the oven. Drain and set aside.
In a large saucepan, melt the 3 tablespoons butter over medium heat. Once melted, whisk in the flour. Cook, stirring constantly, until golden brown.
Slowly whisk in the milk while constantly stirring until combined and smooth. Stir in the salt, dry mustard, garlic powder, and nutmeg, then season pepper. Cook, stirring often, until thickened. Remove from heat and stir in the shredded cheese until smooth.
In a small bowl, beat together the egg yolks. While constantly stirring, pour the egg yolks into the cheese sauce. Mix well. Fold in the cooked macaroni.
In a medium bowl, beat together the egg whites until stiff peaks form. Gently fold into the macaroni and cheese until just combined.
Fill each prepared soufflé ramekins nealy to the top. Arrange on a baking sheet and cook in preheated oven until golden and puffed, 25 to 30 minutes.
Serve immediately with marinara sauce and fresh parsley if desired.