Cook Pasta Slightly: Boil the pasta in salted water for about 4–5 minutes, just until slightly softened but not fully cooked. Drain and set aside.
Prepare the Crock Pot Base: In your slow cooker, combine the milk, sour cream, shredded cheese, butter, cheddar cheese soup, salt, pepper, dry mustard, and cayenne (if using). Stir until mixed.
Add Pasta: Fold the partially cooked pasta into the cheese mixture, ensuring everything is coated evenly.
Cook: Cover and cook on low for 2–3 hours, stirring halfway through. The pasta should be tender and the cheese sauce creamy.
Serve: Stir well before serving. Garnish with extra cheese, parsley, or even a sprinkle of paprika if desired.