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Prepare the Pan
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Preheat oven to 350°F (175°C).
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Grease a 9×13-inch baking dish.
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Make the Batter
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In a large bowl, combine cake mix, eggs, oil, and sour cream.
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Mix until smooth and fully blended (about 1–2 minutes).
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The batter will be thick.
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Create the Cinnamon Layer
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In a small bowl, mix brown sugar and cinnamon.
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Spread half of the cake batter evenly into the prepared pan.
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Sprinkle the cinnamon sugar mixture evenly over the batter.
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Spoon remaining batter on top and gently spread to cover.
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Swirl the Cake
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Use a knife to lightly swirl through the layers to create a marbled effect.
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Don’t overmix — you want visible cinnamon ribbons.
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Bake
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Bake for 35–40 minutes, or until a toothpick inserted in the center comes out clean.
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The top should be golden and slightly firm.
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Make the Glaze
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In a bowl, whisk powdered sugar, milk, and vanilla until smooth and pourable.
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Adjust milk (1 teaspoon at a time) if needed for desired consistency.
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Glaze & Serve
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Pour glaze over the cake while still slightly warm.
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Spread evenly and allow to set for 10–15 minutes before slicing.
