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1
Make the apple butter: in a large pot, stir the apples, lemon juice, sugar, salt, ginger, cider, and cider vinegar to combine. Heat over medium heat until the mixture begins to simmer - continue to cook, stirring occasionally, until the apples soften, 20-25 minutes.
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2
Puree the mixture using an immersion blender, blender, or food processor until smooth. Return to the pot and continue to cook over medium low heat, stirring occasionally, until the mixture reduces slightly and thickens. It should be thick enough that when you dip it into the apple butter, it will fully coat a spoon without dripping off (and it will continue to thicken as it cools).
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3
Transfer to a bowl, cover, and refrigerate until well chilled, at least two hours.
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4
Make the cake: preheat the oven to 325u00b0F. Grease two 8 inch cake pans with nonstick spray, line the bottom with a circle of parchment paper, then flour the sides of the pan.
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5
In a medium bowl, whisk the flour, baking powder, salt, ginger, and nutmeg to combine. In another medium bowl, whisk the egg yolks, vegetable oil, and sugar until pale and thick, about 1 minute. Add the milk and vanilla and whisk to combine.
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6
Gradually whisk the flour mixture into the yolk mixture, mixing just until combined.
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7
In the bowl of an electric mixer fitted with the whisk attachment, whip the egg whites and cream of tartar to medium peaks, 3-4 minutes.
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8
Add about 1/3 of the egg whites to the batter and mix with a silicone spatula to combine. Add the remaining whites in 2-3 additions, folding gently just until incorporated.
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9
Divide the batter between the two prepared pans and spread into an even layer. Sprinkle the surface of each cake with half of the crystallized ginger.
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10
Bake the cakes until they pull away from the edges slightly and a tester inserted into the center comes out clean, 25-30 minutes.
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11
Cool for 15 minutes inside the pan, then invert onto a cooling rack to cool completely.
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12
When the cake is cool, place one cake layer (ginger side up) on a platter or cake stand. Scoop some of the apple butter on top, and spread into an even layer. Top with another cake layer (ginger side up). Frost the top and sides with the remaining apple butter. Serve immediately.
