Heat oven to 350° F. Prepare a deep sided 9×13 pan by coating with cooking spray. Set aside.
Bring a large pot of water to a boil. Add salt, and cook elbow macaroni according to package directions until tender. Drain and transfer the macaroni noodles to the baking pan.
While the macaroni is cooking, combine the milks and eggs in a large bowl. Whisk until thoroughly combined. Add the butter, cheeses, sour cream, salt, pepper, and sugar and stir to combine. Pour over the macaroni and stir to combine. Top with the grated mild Cheddar cheese.
Bake for 30 to 45 minutes or until the top is lightly golden brown