Preheat oven to 400°F with rack in upper third. Butter a 9-inch shallow baking dish.
Briskly simmer cranberries, Port, whiskey, and cinnamon in a small heavy saucepan, uncovered, stirring occasionally, until almost syrupy, 12 to 15 minutes. Pour into baking dish.
Blend together eggs, milk, butter, sugar, flour, vanilla, and salt in a blender and pour over cranberries, then sprinkle with cinnamon sugar. Bake until slightly puffed and set in center, about 35 minutes. Cool clafoutis briefly on a rack.
