Serve this classic, highlighted by goat cheese and scallions, as a main course or a side dish.
Heat oven to 375 degrees. Lightly butter a 2-quart casserole. Set aside. Cook macaroni in a large pot of boiling water according to package instructions. Drain, rinse with cool water to stop the cooking, and set aside.
Warm milk in a medium saucepan over medium-low heat. Melt butter in a large heavy pot over medium heat. Gradually whisk in flour and cook for 2 minutes, or until mixture is thick and smooth. Whisking constantly, gradually add warm milk. Cook over medium-low heat 8 to 10 minutes, whisking occasionally. Remove white sauce from heat; add salt, pepper, and cayenne.
In a bowl, combine 1/4 cup each yellow and white cheddar and 1/4 cup Parmesan; set aside. Set aside 1 tablespoon scallions. Add remaining 1 3/4 cups of both cheddars and 3/4 cup Parmesan to warm white sauce. Stir well. Stir in remaining scallions, then the cooked macaroni. Add goat cheese in 1-inch pieces; fold gently to combine.
Transfer mixture to casserole; top with reserved scallions and cheeses. Bake until golden brown and bubbling, 30 to 35 minutes.
Choose sharp, full-flavored cheeses to give this dish the most impact.