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1
Position a rack in the lowest part of the oven and preheat the oven to 350F.
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2
Use the back of a spoon to smear the 4 tablespoons topping butter all over the bottom of the pan.
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3
With the same spoon, spread the brown sugar over the butter (the brown sugar should be in an even layer but does not need to be incorporated into the butter).
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4
Sprinkle with the cinnamon.
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5
Peel, quarter, and core the apple and cut it into 1/4-inch slices.
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6
Place in a bowl and toss gently with the lemon zest and juice.
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7
Place the apple slices flat in the pan, covering most of the brown sugar layer, and pour the lemon juice from the bowl on top; set aside.
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8
For the cake, combine the rice and oat flours, sugar, butter, and salt in the bowl of the stand mixer and mix on medium speed with the paddle attachment until the mixture is the texture of brown sugar, about a minute.
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9
Add the baking powder, baking soda, xanthan gum, yogurt, eggs, and vanilla and beat on medium-high speed for 2 to 3 minutes; the batter should be very smooth and fluffy.
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10
Scrape into the prepared pan and spread it evenly.
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11
Bake for 40 to 45 minutes, until a toothpick inserted in the center comes out clean.
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12
Let the cake sit for 5 minutes on a rack, then slide a slim knife or small metal spatula around the edge to detach it from the pan.
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13
Invert the cake onto a plate to cool.
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14
If some of the apples stick to the pan, use a spatula to transfer them back into place--the gooey topping will hide all sins here.
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15
Serve wedges with ice cream or whipped cream.
