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1
In a medium-size bowl, place the flour, xanthan gum, salt, granulated sugar and brown sugar. Whisk to combine well, working out any lumps in the brown sugar. Add the oats, and mix to combine. Create a well in the center of the dry ingredients and add the melted and cooled butter. Mix to combine. The mixture should come together and be thick. Place in the refrigerator to chill while you make the cake batter.
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2
Preheat your oven to 350u00b0F. Line a rimmed baking sheet with parchment paper. Grease well a 9-inch round baking pan with at least 2-inch high sides and set it on top of the lined baking sheet.
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3
First cook the prepared apples. In a medium-size, heavy bottom saucepan, place the apple wedges, ground cinnamon and brown sugar, and toss to coat the apples in the cinnamon and sugar. Add 2 tablespoons of water, and mix to combine. Place the saucepan over medium-high heat and cook, stirring occasionally, until the apple slices are fork-tender (about 5 minutes), adding more water if necessary to prevent the mixture from burning. You don't want to cook the apples down into sauce. Remove from the heat and set aside to cool briefly.
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4
In a separate, medium-size bowl, place the flour, xanthan gum, baking powder and salt, and whisk to combine. Set the bowl aside. In the bowl of your KitchenAid(R) Pro Line(R) Series Stand Mixer fitted with the Flat Beater, place the butter and beat on medium-high speed until light and fluffy. Add the sugar, then the eggs and vanilla, beating to combine well after each addition. Add the dry ingredients to the KitchenAid(R) Pro Line(R) Series Stand Mixer bowl in 3 parts, alternating with the milk, and beginning and ending with the dry ingredients. Mix until just combined. Remove the mixer bowl from the Stand Mixer, and fold the cooked apples into the smooth batter until evenly distributed throughout. Scrape the batter into the prepared cake pan and smooth into an even layer with a wet spatula.
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5
Remove the chilled crumble topping from the refrigerator and, using a fork, break the crumble into loose, irregular pieces. Place the crumble pieces on top of the cake batter in the pan in an even layer and press down gently to help the topping adhere to the cake.
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6
Place the cake pan back on the lined baking sheet, and place in the center of the preheated oven. Bake until the crumble topping is lightly golden brown all over and a toothpick inserted into the center of the cake comes back mostly clean, or with a few moist crumbs attached (about 45 minutes). When it's done, the top of the cake will spring back when pressed gently in the center. Remove from the oven and allow to cool in the pan for at least 30 minutes. Remove any bits of baked crumble topping that are sticking to the edge of the pan. They might make it difficult to remove the cake from the pan. Working quickly, invert the cake onto a plate and then re-invert the cake, so it is crumble-side up, onto a wire rack to cool completely. Slice and serve.
