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Preheat the oven to 350°F. Butter a 9 1/2" to 10" (2" deep) cast iron skillet; or a 9" square cake pan.
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Combine the peeled, sliced apples with the brown sugar, boiled cider, spices, and salt. Set aside.
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Combine the flour, sugar, baking powder, and salt. Set aside.
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Mix the warm milk, egg, melted butter, and vanilla. Add to the flour mixture, stirring to combine. Pour into the prepared skillet.
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Spoon the apple mixture onto the batter. For the best appearance, make sure the apples are distributed a little more heavily towards the edges of the pan. Sprinkle with coarse sparkling sugar, if desired.
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Bake the cake for about 50 to 60 minutes, till it's light brown and a cake tester inserted into the center comes out clean.
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Remove from the oven, and cool for about 5 minutes. Loosen the edges of the cake from the pan, and cool for another 20 minutes or so.
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If you haven't sprinkled with coarse sparkling sugar, dust with confectioners' or glazing sugar, and serve with whipped or iced cream, right from the pan.