Start by bringing a large pot of salted water to a boil. Add the 1 pound of large elbow pasta and cook according to package instructions until al dente. Drain and set aside.
While the pasta is cooking, prepare your slow cooker. You don’t need to grease it, but if you prefer, you can lightly spray it with cooking spray for easier cleanup.
In the slow cooker, add the drained pasta, slivers of butter, pieces of cream cheese, salt, black pepper, cayenne pepper, paprika, evaporated milk, whole milk, and heavy cream. Stir everything together until well combined.
Add in the freshly shredded sharp cheddar cheese and pepper jack cheese. Reserve a small handful of cheese for topping later. Mix everything until the cheeses are evenly distributed throughout the pasta.
Cover the slow cooker and set it to low heat. Let it cook for about 2 to 3 hours, stirring occasionally. The slow cooking will allow all the flavors to meld together beautifully.
Once the cooking time is up, stir the mac and cheese one last time. If you’d like, sprinkle the reserved cheese on top and let it melt under the lid for a few minutes before serving. Enjoy your Slow Cooker Ultra Creamy Mac and Cheese hot!