Spray 3½ to 4-quart slow cooker with cooking spray. Cook the pasta to al dente and drain. Place the macaroni in the slow cooker, immediately add the butter and stir until melted.
Add the evaporated milk, half & half, 2½ cups of the cheddar cheese, the Velveeta, salt and pepper; stir to blend well.
Cover and cook on low for 2 to 3 hours (if you have a larger slow cooker, decrease the cooking time). During the last 15 minutes of cooking, sprinkle with the remaining ½ cup of cheddar cheese. Once done, turn the slow cooker setting to warm until ready to serve. Leftovers can be stored in an airtight container in the refrigerator for up to 4 days.