Prepare the fries according to the package directions. Keep the oven on after they are done baking.
Meanwhile, combine the brown sugar, butter, cream, vanilla and 1/2 teaspoon salt in a medium saucepan. Bring to a boil, stirring to dissolve the sugar. Once the sauce comes to a full and rolling boil, cook for 1 minute more. Remove from the heat and keep the sauce warm.
Whisk together the granulated sugar and cinnamon in a large bowl. Toss the hot baked fries in the cinnamon sugar until evenly coated. Pile them into an 8- to 9-inch heatproof serving dish and scatter the marshmallows over the top. Put the serving dish on a baking sheet and return to the oven. Bake until the marshmallows soften and just start to brown on top, about 5 minutes. Drizzle with the warm brown sugar sauce before serving.
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