Boil a large pot of salted water. Cook the elbow macaroni 1–2 minutes shy of al dente (about 5–6 minutes); drain very well. Toss the hot pasta with 1 tablespoon of the melted butter and set aside.
Preheat the slow cooker on Low and lightly mist the insert with nonstick spray.
In a bowl, whisk evaporated milk, whole milk, remaining melted butter, salt, onion powder, dry mustard, paprika (if using), and black pepper until smooth. Pour into the warm slow cooker.
Stir in the cubed processed cheese and 1 cup of the shredded cheddar. Cover and cook on Low for 45–60 minutes, stirring once halfway, until the cheeses are fully melted and the sauce is silky.
Fold in the warm macaroni and the remaining 1 cup cheddar. Cover and cook on Low for 35–45 minutes more, stirring once or twice, until the pasta is tender and coated in a creamy sauce.
For a thicker finish, cook the last 10–15 minutes with the lid slightly ajar or with a clean kitchen towel under the lid to catch condensation. Taste and season to preference, then serve hot.