Begin by boiling a large pot of salted water over high heat. Once it reaches a rolling boil, add the elbow pasta and cook until al dente, about 6-8 minutes. Keep an eye on it to ensure it doesn't get mushy, then drain the pasta and set it aside in a large mixing bowl.

In a medium skillet, melt 4 tablespoons of unsalted butter over medium heat. Once melted, gradually add the sharp cheddar and mozzarella cheese, stirring continuously until both are fully melted and creamy.

In your slow cooker, combine the melted cheese sauce with the cooked pasta. Add in the sour cream, cheddar cheese soup, milk, salt, mustard powder, and paprika. Stir thoroughly to make sure everything is well combined.

Cover the slow cooker and set it to low heat. Let it cook for 2 hours, stirring occasionally to ensure even cooking and a creamy texture.

Once the cooking time is up, give the mac and cheese a gentle stir before serving. Garnish with fresh parsley or chives if desired, and consider topping with crispy bacon or breadcrumbs.