Bring a large pot of salted water to a boil. Cook the macaroni 1 minute less than the package directions, until just shy of al dente. Drain well.

Grease the inside of a 4- to 6-quart slow cooker with butter or nonstick spray, or use a liner.

Return the hot pasta to the pot. Add the butter and 2 cups of the cheddar. Stir until melted into the noodles.

In a bowl, whisk together the eggs, sour cream, condensed soup (if using), milk, salt, pepper, and cayenne. Pour over the pasta while stirring until fully combined.

Transfer to the slow cooker and sprinkle the remaining ½ cup of cheddar on top. Cover and cook on LOW for 2½ to 3 hours, stirring gently once halfway through. It's done when the center is set and the edges are lightly golden.

Switch to WARM and serve straight from the crock. Let rest 10 minutes if it looks loose; it firms up as it sits.