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Preheat the oven to 350 degrees. Spray a 13- by 9-inch baking dish with cooking spray and set pan aside.
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Combine the cake mix, eggs, sour cream, vegetable oil, Fireball, cinnamon, and salt in a large mixing bowl. Beat with an electric mixer on medium speed for 2 minutes, until smooth.
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Pour into the prepared pan and bake 24 to 30 minutes, until a toothpick inserted into the center of the cake comes out clean.
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Cool cake on a wire rack for 2 hours.
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Once the cake has cooled, use the handle of a wooden spoon to poke holes every inch over the top of the cake, going about three-quarters of the way to the bottom. Pour the sweetened condensed milk over the cake and use a rubber spatula to push it into the holes.
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Set aside while you make the frosting.