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Oven Temperature: 350F
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For Crumb Topping and Filling:
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Preheat the oven to 350F 30 minutes before baking.
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Have an oven shelf at the middle level.
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Make the crumb mixture.
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In a food processor fitted with the metal blade, pulse the sugars, nuts, and cinnamon until the nuts are coarsely chopped.
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Reserve 1/2 cup for the filling.
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Add the flour, butter, and vanilla to the remainder and pulse briefly just until the butter is absorbed.
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Empty it into a bowl and refrigerate for about 20 minutes to firm up, then use your fingertips to form a coarse, crumbly mixture for the topping.
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Prepare the filling and mix the batter.
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Just before mixing the batter, peel and core the apple.
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Cut into 1/4-inch-thick slices, and toss with the lemon juice.
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In a medium bowl, lightly combine the egg, the yolks, about one fourth of the sour cream, and the vanilla.
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In a mixer bowl, or other large bowl, combine the cake flour, sugar, baking powder, baking soda, and salt.
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Mix on low speed (#2 if using a KitchenAid, with the paddle attachment) for 30 seconds to blend.
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Add the butter and the remaining sour cream and mix until the dry ingredients are moistened.
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Increase the speed to medium if using a stand mixer (#4 KitchenAid), or high speed if using a hand-held mixer, and beat for 1 minute to aerate and develop the structure.
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Scrape down the sides of the bowl.
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Gradually add the egg mixture in two batches, beating for 20 seconds after each addition to incorporate the ingredients and strengthen the structure.
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Scrape down the sides.
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Fill the pan.
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Scrape about two-thirds of the batter into the prepared pan.
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Smooth the surface.
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Sprinkle with the reserved 1/2 cup crumb mixture and top with the apple slices, arranging them in two rows of overlapping slices.
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Drop the reserved batter in large blobs over the fruit and spread it evenly, preferably with a small offset spatula.
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(The batter will be 3/4 inch from the top of the pan.)
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Sprinkle with the crumb topping.
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Bake the bread.
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Bake for 50 to 60 minutes or until a wooden toothpick inserted in the center comes out clean and the bread springs back when pressed lightly in the center (an instant-read thermometer inserted into the center will read about 200F).
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Tent loosely with buttered foil after 45 minutes to prevent overbrowning.
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Cool the bread.
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Remove the bread from the oven and set it on a wire rack to cool for 10 minutes.
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Place a folded kitchen towel on top of a flat plate and cover it with plastic wrap.
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Oil the plastic wrap.
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Loosen the sides of the bread with a small metal spatula, and invert it onto the plate.
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Grease a wire rack and reinvert the bread onto it, so that it is right side up.
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Cool completely, about 1 1/2 hours, before wrapping airtight.
