Creamy Macaroni and Cheese

INGREDIENTS

• 4 tablespoons (1/2 stuck) butter, cut into pieces
• 1 cup whole milk
• 1 (10 3/4-ounce) can condensed Cheddar cheese soup
• 3 eggs, beaten
• 2 cups uncooked elbow macaroni (an 8-ounce box isn't quite 2 cups)
• 1/2 teaspoon salt
• 1/2 teaspoon dry mustard
• 2 1/2 cups (about 10 ounces) grated sharp Cheddar cheese
• 1/2 cup sour cream
• 1/2 teaspoon black pepper