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1
Heat the oven to 375 degrees.
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2
Place 3/4 cup of sugar in a cast-iron skillet and cover with 1/4 cup of water.
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3
Stir to make sure all of the sugar is wet and bring to a boil over medium-high heat.
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4
Keep the mixture covered until the syrup is clear and producing bubbles for 3-4 minutes, stirring occasionally.
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5
Uncover and cook until the sugar is a light butterscotch color and its temperature reaches 320 degrees, stirring constantly, then turn off the heat.
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6
When the temperature reaches 350 degrees, whisk in the butter, one piece at a time.
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7
Make sure each piece is incorporated before adding the next, then add the salt.
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8
Cover the caramel with the sliced potatoes, starting in the center and overlapping in a spiraling outward circle as you go.
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9
Cook over low heat for 15 minutes, stirring occasionally to make sure the sweet potatoes are cooking evenly and not sticking, but be sure to return the pieces to the original configuration.
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10
Cover the pan with the puff pastry.
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11
Tuck in the edges around the potatoes and prick several times with a fork.
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12
Bake until the edges are light brown and the pastry is puffed and golden, 40 to 45 minutes.
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13
Remove from the oven and cool for 10 minutes before inverting the pan onto a large plate, then cool for another 20 minutes before slicing and serving.
