Spray your slow cooker with nonstick spray to make for easier clean up.
Bring a large pot of water to a boil and cook the macaroni for 6 minutes. Drain and add to a 6 quart slow cooker.
Add the shredded cheddar and butter to a microwave safe bowl and heat in 30 second increments, stirring each time, until melted. This mixture will be thick and greasy, but it will come together as it cooks in the slow cooker. Pour the cheese mixture over the macaroni.
Add the cream cheese, cheddar soup, sour cream, milk, salt, pepper, and mustard powder to the slow cooker and stir well to coat the noodles.
Cover and cook on low for 3 hours.
Stir well before serving.
Video
Notes
The cheddar and butter won't melt into a traditional smooth/creamy sauce in the microwave. It will be a bit greasy and gloppy. Just trust the process and it will all come together in the slow cooker.You'll find the condensed cheese soup near the rest of the condensed soups in the grocery store.Every slow cooker cooks a bit differently and some run hotter than others. If this is done before you're ready to serve, just switch the slow cooker to the 'warm' setting and keep covered.If you'll want to leave this on the warm setting for any length of time, I recommend stirring well and adding a splash of milk to the pasta so it doesn't dry out.