Prepare a slow cooker with a 4 to 6-quart capacity by spraying it with cooking spray. Cook the elbow macaroni just until it’s al dente, then drain the pasta. Place the hot macaroni directly into the slow cooker and immediately mix in the butter, stirring until it’s fully melted.
Next, pour in the evaporated milk, half & half, and whole milk. Add 3½ cups of the shredded cheddar cheese, the cubed Velveeta, salt, and pepper. Stir thoroughly to ensure all the ingredients are well combined.
Cover the slow cooker and cook on a low setting for 2 to 3 hours. About 15 minutes before the cooking time is up, sprinkle the remaining ½ cup of cheddar cheese over the top. Once the macaroni and cheese is finished, switch the slow cooker to the warm setting until you're ready to serve. Any leftovers should be stored in an airtight container in the refrigerator, where they will keep fresh for up to 4 days.