Prepare the Slow Cooker:
Lightly grease the inside of your slow cooker with nonstick cooking spray or butter to prevent sticking.
Combine Ingredients:
In a mixing bowl, whisk together the melted butter, whole milk, evaporated milk, garlic powder, mustard powder (if using), salt, and black pepper.
Assemble in the Slow Cooker:
Add the uncooked macaroni to the slow cooker. Pour the milk mixture over the pasta, stirring to combine.
Add Cheese:
Stir in the shredded cheddar, mozzarella, and Parmesan cheeses, ensuring the pasta is well-coated.
Cook:
Cover and cook on low for 2–3 hours, stirring every 30 minutes to ensure even cooking and prevent the cheese from clumping.
Check for Doneness:
After about 2 hours, check the macaroni. It should be tender, and the cheese sauce should be smooth and creamy. Adjust seasoning if needed.
Serve:
Serve hot as a main dish or side, garnished with additional cheese or fresh parsley if desired.
