Spray a 10 inch cake pan with nonstick cooking spray.
In a small saucepan bring the maple syrup to a boil over high heat then lower the heat to a simmer to reduce it to approximately ¾ cup. Allow it to cool slightly and then pour it into the greased cake pan.
Arrange the apple slices over the reduced syrup. Scatter the walnut halves over the apples. Set aside.
In a medium mixing bowl whisk together the flour, baking powder, baking soda and salt until well blended.
In the bowl of an electric mixer using the paddle attachment, cream the butter and sugar until light and fluffy.
In a liquid measuring cup, combine the eggs, buttermilk and vanilla extract. Blend together with a small whisk or fork to break up the eggs.
On low speed add approximately 1/3 of the dry ingredients followed by half of the liquid ingredients, stopping the mixer and scraping down the sides of the bowl with a rubber spatula occasionally.
Add another 1/3 of the dry ingredients followed by the remainder of the wet ingredients. Add the remaining dry ingredients and blend well.
Pour the batter over the apples and walnuts.
Bake approximately 1 ½ hours or until nice and brown on top and a small knife inserted into the center comes out clean. Check the cake for doneness after 1 hour and 20 minutes.
Cool for about 30-45 minutes and flip out onto a platter. Gently replace any apples that may have stuck to the bottom of the pan back onto the cake.