Preheat oven to 400 degrees F. Lightly coat a 2-quart rectangular baking dish with cooking spray; set aside. For topping, in a small bowl combine panko and the 2 tablespoons melted butter; set aside.
In a large saucepan heat the 1/4 cup butter over medium heat until melted. Add flour; cook and stir for 2 minutes. Gradually stir in evaporated milk. Cook and stir until mixture is thickened and bubbly. Stir in salt, white pepper, and, if desired, cayenne pepper. Stir in spinach.
Spread half of the spinach mixture in the prepared baking dish. Top with half of the cheese. Repeat layers. Sprinkle with topping. Bake, uncovered, for 20 to 25 minutes or until cheese is melted and topping is golden. Let stand for 5 minutes before serving.