In a medium saucepan, melt butter and flour until browned. Slowly whisk in milk. Add salt and cayenne. Cook until boiling and reduce heat and stir in cheese. In a greased 9x13 casserole dish, layer half the potatoes. Pour half the cheese sauce over top. Layer with remaining potatoes. Add remaining cheese sauce. Top with cheese and Panko breadcrumbs. Bake 350F for an hour.