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1
Make topping: Peel, core, and quarter apples.
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2
In a well seasoned 10 1/4 by 2-inch cast iron skillet melt butter over moderately low heat just until melted (butter should not separate).
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3
Stir in sugar until combined well.
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4
Arrange apple quarters decoratively, cut sides up, in skillet and sprinkle walnuts evenly between apples.
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5
Cook mixture, undisturbed, 25 to 35 minutes, or until the apples are tender in centers and sugar is a golden caramel.
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6
Preheat oven to 375 degrees.
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7
Make cake batter while topping is cooking: Peel apple and chop fine.
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8
In a bowl whisk together flour, baking powder, salt, and cinnamon.
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9
In another bowl with an electric mixture beat butter and sugar until light and fluffy.
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10
Beat in vanilla and gingerroot and add eggs, 1 at a time, beating well after each addition.
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11
Beat in sour cream and with mixer on low speed beat in flour mixture gradually until just combined.
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12
Fold chopped apple into batter.
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13
Remove skillet from heat and spoon batter evenly over topping.
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14
With a metal spatula spread batter (being careful not to disturb topping), leaving a 1/4-inch border of cooked apples uncovered.
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15
If using a non-stick stick skillet with a plastic handle, wrap handle in a double thickness of foil.
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16
Put skillet in a shallow baking pan and bake cake in middle of oven 25 to 35 minutes, or until a tester comes out with crumbs adhering and cake is golden brown.
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17
Cool cake in skillet on a rack 10 minutes.
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18
Run a thin knife around edge of skillet and carefully invert cake onto a plate.
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19
Serve cake warm or at room temperature with caramel sauce and whipped cream.
