Spray the insert of a 6-quart slow cooker with non-stick cooking spray to prevent sticking.
Add the uncooked cellentani pasta, whole milk, heavy cream, kosher salt, and cayenne pepper to the slow cooker. Stir well to combine.
Distribute the cubed butter and cream cheese pieces evenly over the top of the pasta mixture. Do not stir.
Cover and cook on LOW for 1 hour. After 1 hour, remove the lid and stir the pasta to ensure it cooks evenly.
Continue cooking on LOW for another 15 to 30 minutes, or until the pasta is tender (al dente).
Once the pasta is cooked, stir in the freshly shredded cheddar and pepper jack cheeses. Stir constantly until the cheese is completely melted and the sauce is smooth. Serve immediately.