Add the uncooked cellentani pasta to a 6-quart slow cooker.
Top the pasta with the butter slices, cream cheese cubes, kosher salt, and cayenne pepper.
Pour in the whole milk and heavy cream, then stir gently to combine everything and ensure the pasta is spread in an even layer.
Cover and cook on Low for 1 hour. Stir the mixture, then continue cooking for another 15 to 45 minutes until the pasta is al dente (tender but firm).
Add the freshly shredded cheddar and pepper jack cheeses to the slow cooker.
Stir well to combine, then cover and cook for an additional 10 minutes to fully melt the cheese into a creamy sauce.
Stir once more and serve immediately.