Bring a large pot of water to a boil over high heat; add macaroni and 4 1/2 teaspoons salt. Cook pasta until just barely tender, about 5 minutes. Drain and rinse with cold water to stop the cooking process.

Spray the inside of a slow cooker with cooking spray. Add pasta to the slow cooker and stir in milk, evaporated milk, butter, cream cheese and pepper. Whisk together egg and egg yolk in a small bowl until well combined; stir into pasta mixture. Set aside 3/4 cup shredded cheese. Stir remaining cheese into pasta mixture, cover, and cook on Low until bubbly and lightly browned around the edges, about 1 1/2 hours.