Cook the elbow macaroni in heavily salted water according to the package instructions until al dente. The noodles will continue to soften as the mac and cheese bakes. Drain well.
Preheat the oven to 350°F. While the macaroni is still hot, add the salted butter, sour cream, and about half of the combined mild cheddar, medium cheddar, Colby Jack, and mozzarella. Mix well until the butter and cheese begin to melt and everything is evenly incorporated.
Add the onion powder, garlic powder, paprika, salt, white pepper, and mustard powder. Mix well again.
In a separate bowl, whisk together the eggs, evaporated milk, and ½ cup of heavy cream to make the custard.
Gradually pour the custard into the macaroni and cheese, mixing between additions, until you reach a thick but creamy consistency that is moist without being too soupy. You may have a few tablespoons of custard remaining, and that’s OK. Alternatively, if you prefer a creamier consistency and need more liquid, gradually add a few additional tablespoons of heavy cream until it looks right to you.
Add about one-third of the mac and cheese to a 9x 13-inch casserole dish, spreading it evenly across the bottom. Sprinkle about one-third of the remaining shredded cheese over the first layer. Repeat the layering process two more times, finishing with the remaining cheese on top.
Bake uncovered at 350°F for approximately 35 minutes, or until the custard is set and the mac and cheese is bubbly and golden to your liking. For a little more color and a slight crust on top, switch the oven to low broil for the final 3 to 5 minutes, watching closely to prevent burning.
Remove the mac and cheese from the oven, and allow it to rest for at least 10 minutes before serving.