Preheat oven to 375 degrees F (190 degrees C). Line a 9x13-inch baking dish with parchment paper.
Bring a large pot of water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until almost cooked through and firm to the bite, about 7 minutes. Drain and transfer to a large bowl. Sprinkle macaroni with 1/2 teaspoon salt and stir 1/2 cup butter into the pasta.
Mix 1/4 cup butter, sour cream, cream cheese, sharp Cheddar cheese, and egg yolk together in a bowl. Stir flour, 1/2 teaspoon salt, cayenne pepper, and milk into the sour cream mixture.
Spread 1/4 cup sour cream sauce over bottom of prepared baking dish. Stir remaining sour cream sauce into macaroni. Pour macaroni into baking dish atop sauce layer; sprinkle mild Cheddar cheese over the casserole.
Bake in the preheated oven until heated through and cheese topping has melted, about 15 minutes.
