Preheat oven to 400 degrees with rack in upper third. Butter a 9-inch shallow baking dish.
In a small heavy saucepan, briskly simmer cranberries, Port, whiskey, and cinnamon, uncovered, stirring occasionally, until almost syrupy, 12 to 15 minutes. Pour into buttered baking dish.
In a blender, blend together eggs, milk, butter, sugar, flour vanilla, and salt. Pour over cranberries, then sprinkle with cinnamon sugar.
Bake until slightly puffed and set in center, about 35 minutes. Cool briefly on a rack before serving. Serve with vanilla ice cream or whipped cream