Butter a springform cake pan and line with parchment paper.
In a large bowl, using an electric or stand mixer, cream together the butter and maple sugar until light and fluffy. With mixer on low, add the eggs one at a time, beating well to incorporate fully. Add the sour cream and vanilla and mix until smooth.
In a medium bowl, whisk together the flours, baking powder, baking soda, and salt. Keep the mixer on low and slowly add the dry mix to the batter. Mix until well combined.
In a small bowl, stir together the coconut sugar and cinnamon.
Spread half of the batter in the prepared pan, smoothing it with a rubber spatula. Top the batter with one-half of the sliced apples and sprinkle with two-thirds of the coconut sugar/cinnamon. Spread the remaining batter on top, again using the spatula to smooth the top. Place the remaining apples on top and sprinkle with the remaining sugar mixture.
Preheat oven to 350℉. Let the batter rest and hydrate in the pan while the oven heats. Then place the cake in the oven and bake for 55-65 minutes, checking frequently after 55 minutes. Bake until a toothpick inserted in the center comes out clean, the top of the cake is set and slightly springs back when pressed, and the edges are pulling away from the sides of the pan. Remove from oven and cool in the pan on a wire rack for 20 minutes, then turn out onto the rack to cool completely.
Cake will keep, well wrapped in the fridge, for up to 5 days. I find it tastes best at room temperature or slightly warmed.