1) Preheat the oven to 350°F. Lightly grease a 9" round cake pan at least 2" deep; or a 9" springform pan.
2) Spread the walnuts in a single layer in an ungreased pan, and bake them for 8 to 10 minutes, until they're a light golden brown. Remove them from the oven, and sprinkle very lightly with salt. Set them aside.
3) Beat together the sugar, butter, cinnamon, baking powder, salt and vanilla till smooth.
4) Add the eggs. Beat till thoroughly combined.
5) Stir in the sour cream or yogurt alternately with the flour, using the low speed of your mixer. The batter will be stiff.
6) Spoon the batter into the prepared pan, smoothing the top with a spatula as best you can.
7) Toss the apple wedges with the sugar and vanilla till apples are thoroughly coated.
8) Arrange the apples atop the cake, pushing them into the stiff batter gently so they're partially submerged.
9) Bake the cake for 65 minutes, till it's golden brown on top and a toothpick or cake tester inserted into the center comes out clean.
10) Remove the cake from the oven, and turn it out onto a rack, apple-side up.
11) Place the caramel and cream in a microwave-safe bowl, and heat till the cream is showing bubbles, and the caramel is soft.
12) Stir the caramel and cream till smooth.
13) Spoon the caramel over the cake, dripping it down into the crevices around the apples.
14) Scatter the walnuts on top. Serve warm, or at room temperature.