Add 1/4 cup melted butter to a 4 quart slow cooker (a larger slow cooker is fine*). Use a pastry brush to coat the bottom and sides of the slow cooker with the butter.
Add 2 cups (9 oz) uncooked elbow macaroni to the butter and stir to coat.
Add a 12-ounce can evaporated milk, 1 cup whole milk, 1/2 cup cream, 1/2 cup mayonnaise, 1 teaspoon salt, 1/2 teaspoon pepper, 1/8 teaspoon cayenne, and stir the mixture well.
Chop your American cheese into 1 inch chunks. (Buy the American cheese from behind the counter of the deli at your store. You can substitute Kraft singles or Velveeta in a pinch, but it is not as good.) Add to the slow cooker.
Shred 6 ounces of sharp cheddar cheese. Do not use pre shredded cheddar cheese. It does not melt as well, and will make your mac and cheese less creamy. Shred the cheese from a block, and if you are measuring in cups, make sure you pack the cheese in the cup to get the proper measurement. Add the cheese to your cup and smash it in with your fist until you can fit any more in. 1 and 1/2 cups well-packed shredded cheese will be about 6 ounces. Add to the slow cooker.
Add 2 ounces (1/2 cup) shredded parmesan cheese to the slow cooker. Same rules apply here: Do not use pre shredded cheese. Pack the cheese into the 1/2 cup measuring cup with your fist until you can't fit anymore in.
Once all the cheese is added in, stir the mixture together. Turn the heat to the LOW setting and put on the lid.
Stir after 1 hour, moving quickly to keep as much of the heat in as you can. Focus on scraping the bottom and edges well. Eventually the cheese will start to form a crust on the outer edges. I love these cheesy crunchy bits, and stir them into my creamy mac and cheese, but if you want a very smooth mac and cheese, just avoid scraping the edges too much once the crust has formed. Put the lid back on and cook on low for another 30-60 minutes. If your macaroni seemed like it was close to being cooked after the first stir, you will want to check it again after 30 minutes. If it seemed like it had a long way to go, check after 60 minutes. Keep the lid on as much as possible. The macaroni and cheese is done when the noodles are al dente and the sauce is creamy and smooth. If you like a thicker texture, simply leave it in the crock pot with the lid on for a few more minutes. The mac and cheese will continue to thicken as is cools.
Once it is cooked, turn the setting on your slow cooker to "WARM." This will keep your mac and cheese nice and creamy. Serve immediately, or leave on the warm setting for up to 2-3 hours, stirring every 30-60 minutes. Add a bit of cream and stir if the mixture starts to get too thick for your liking.
If you love EXTRA cheesy mac and cheese (me! me!) Take a few slices of cheese (any kind you like) and tuck them into the center of the crock pot. Use a spoon to shove some of the mac to one side, place a few slices down in the middles, then cover with the mac and cheese. See photos! It will melt and create an amazing cheese pull when you serve it! You could also add shredded cheese to the top, or a layer of sliced cheese. Never enough!!
Storage: This mac and cheese keeps well in the fridge for 2-3 days. It also freezes well in an airtight container. Thaw completely in the fridge before reheating.
Reheating: Leftover slow cooker mac and cheese is tasty, but it will never have the same creamy texture that it had when you first made it. The microwave is a great option for reheating; pour off any grease that forms, or absorb with paper towels (this is normal when cheese separates, and can't really be avoided.) Leftovers are best when only reheated once; heat individual servings when possible. You can also add the entire mixture back into the slow cooker and heat on low, stirring often, being prepared to absorb grease that forms.