Coat the inside of a 6-quart slow cooker with nonstick cooking spray. Place the uncooked elbow macaroni in a colander and rinse it thoroughly beneath cold running water. Drain well before proceeding. Rinsing away some of the surface starch helps reduce gumminess as the pasta cooks.
Transfer the rinsed and thoroughly drained macaroni to the prepared slow cooker. Pour in the whole milk and evaporated milk.
Add the freshly shredded extra-sharp cheddar and American cheese. Season the mixture with salt, black pepper, dry mustard, garlic powder, and a dash of cayenne pepper according to taste.
Stir everything thoroughly so the cheese, seasonings, and liquids are evenly distributed. Arrange the macaroni so that as much of the pasta as possible remains beneath the liquid for more consistent cooking.
Distribute the cubed butter evenly across the surface of the macaroni mixture. Cover the slow cooker securely with its lid and cook on the low setting for 1 hour.
After the first hour, briefly uncover the slow cooker and stir the macaroni well, bringing the melted cheese and sauce together while redistributing the pasta. Replace the lid promptly to minimize heat loss.
Test the macaroni for tenderness. Some slow cookers may cook the pasta sufficiently within the first hour. If the noodles are still too firm, cover the cooker again and continue cooking on low.
Check and stir the macaroni every 30 minutes until the pasta reaches the desired tenderness. Tasting a piece during each check can help prevent the noodles from becoming overcooked.
When the macaroni is tender and the cheese sauce has developed a thick, creamy consistency, remove the lid. Allow the finished mac and cheese to stand for a few minutes so the sauce can thicken slightly before serving.
Serve while hot. If desired, finish each serving with a small additional sprinkling of cheddar cheese.