Boil the macaroni in water with 1 teaspoon salt for 3 minutes less than the package directs, drain it, toss with the butter, and cool for 10 minutes.
Whisk the eggs, evaporated milk, cottage cheese, mustard, pepper and remaining salt.
Coat a 6-quart crockpot with butter and layer half the macaroni, 2 cups cheddar, the remaining macaroni and 2 more cups cheddar.
Pour the egg mixture evenly over the layers, press the pasta under the liquid, and scatter the remaining 1 cup cheddar over the top.
Cover and cook on low for 2 to 2 1/4 hours until the edges are set, the center reaches 160°F and a spoonful holds together; rest uncovered for 15 minutes before serving.