Preheat oven to 350 F. Butter and flour a 10 inch round cake pan.
In a medium saucepan, bring the maple syrup to a boil and then reduce heat and simmer for about 15-20 minutes or until maple syrup is reduced to 3/4 cup.
Pour the reduced maple syrup into the cake pan and covering the bottom completely with the maple syrup. Arrange the apple slices on top of the maple syrup in a concentric circle, overlapping the slices. Repeat with an inner circle. Fill the pan completely with apple slices.
In a large bowl whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside. In a small bowl, whisk together the eggs, buttermilk and vanilla.
In a bowl with an electric mixer, beat the butter and sugar together until light and fluffy, about 3 minutes. In 3 alternating batches, beat the dry ingredients followed by the wet ingredients into the butter/sugar mixture.
Pour the batter over the apple slices and smooth evenly. Bake for 90 minutes or until the top is golden and a toothpick comes out clean. Cool the cake on a rack for 45 minutes.
Run a knife around the cake and invert it on to a serving plate. Serve with creme fraiche, whipped cream or vanilla ice cream.