Cook the elbow pasta according to the package directions.
Drain the cooked pasta thoroughly and set it aside.
While the pasta is cooking, place a large skillet over low to medium-low heat.
Add the butter, shredded cheddar cheese, and shredded mozzarella cheese to the skillet.
Stir continuously until the butter has melted and the cheeses begin to melt into a smooth mixture.
Transfer the melted cheese and butter mixture to the slow cooker.
Add the cheddar cheese soup and sour cream to the slow cooker.
Stir until the ingredients are evenly combined.
Add the cooked elbow pasta to the slow cooker.
Pour in the milk.
Add the salt, mustard powder, and paprika.
Stir all ingredients until the pasta is evenly coated with the cheese mixture.
Cover the slow cooker with the lid.
Cook on low for 2 hours, stirring occasionally if possible.
Check the mac and cheese near the end of the cooking time to ensure the pasta is tender and the sauce is creamy.
If the mixture becomes too thick, stir in a small amount of additional milk until the desired consistency is reached.
Once fully heated and creamy, stir gently before serving.
Serve warm.