Preheat the oven to 375° F. Butter a springform pan and line the bottom with a circle of parchment paper.
Mix together the walnuts, the cinnamon, and 2 tablespoons of the sugar in a bowl and set aside.
In the bowl of a stand mixer (or using hand beaters), beat together the butter and 1 1/2 cups sugar until light-colored and fluffy.
Add the eggs and the vanilla and beat until combined.
Sift the dry ingredients together (you will have sifted the flour twice, which keeps the cake light even with the addition of the sour cream).
With the mixer running at low speed, alternate adding the dry ingredients and the sour cream to the butter and sugar mixture. You want 3 additions of each, ending with the sour cream. Be sure not to over-mix!
Use a wooden spoon or a spatula to fold in the chocolate chips.
Pour half of the batter into the prepared springform pan. Starting on the outside edge, place the apple slices in a concentric circle. Sprinkle half of the cinnamon-sugar-walnut mixture over the apples, then pour in the rest of the batter. Smooth with a rubber spatula.
Top the cake with the remainder of the cinnamon-sugar-walnut mixture, then bake it for 40 minutes, or until it's golden-brown and a toothpick inserted in the center comes out clean. Let the cake cool in the pan until the edges start to pull away from the sides.