Heat oven to 350 degrees F. Coat a 3-quart baking dish with nonstick cooking spray.
Cook macaroni following package directions. Drain and return to pot.
While pasta cooks, combine milk and flour in a saucepan; whisk until smooth. Cook over medium heat, whisking constantly, until it boils and thickens. Remove from heat. Add cheddar, whisking until smooth. Whisk in cottage cheese, dry mustard, salt and cayenne.
Stir cheese sauce into pasta. Gently fold in baby spinach.
Spoon mixture into prepared baking dish. Combine bread crumbs and Parmesan. Sprinkle on top.
Bake at 350 degrees F for 25 to 30 minutes. Remove from oven; let stand 10 minutes.