Bring a large pot of salted water to a boil. Add the elbow macaroni and cook for 3 minutes less than the package directions for al dente. Drain well.
16 oz elbow macaroni
Place the partially cooked macaroni in a 6-quart slow cooker. Add the milk, Velveeta, butter and cheddar cheese. Stir to combine.
3 cups whole milk, 8 oz Velveeta cheese, 1/4 cup salted butter, 3 cups sharp cheddar cheese
Sprinkle the Parmesan cheese evenly over the top.
1/4 cup Parmesan cheese
Cover and cook on Low for 2 to 3 hours, stirring occasionally. Replace the lid promptly after stirring to prevent heat from escaping.
Begin checking for doneness at the 2-hour mark. Remove from the heat as soon as the noodles are tender and the cheese sauce is completely melted and creamy. Serve immediately.