2large applespeeled, cored, cut into 8 wedges each
Preheat oven to 350.
Lightly grease a 9" round cake pan at least 2" deep; or a 9" springform pan.
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 from the oven, and sprinkle very lightly with salt. Set aside.
Beat together the sugar, butter, cinnamon, baking powder, salt and vanilla until smooth.
Add the eggs. Beat till thoroughly combined. Stir in the sour cream or yogurt alternately with the flour, using the low speed on mixer. The batter will be stiff. Spoon the batter into the prepared pan, smoothing the top with a spatula, don't be concerned if it's not perfect, just try to spread it evenly.
Toss the apple wedges with the sugar and vanilla till apples are thoroughly coated.
Arrange the apples on top of the cake batter, pushing them into the batter gently so they're partially submerged.
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.
Remove the cake from the oven, and turn it out onto a rack, apple-side up.
Place the caramel and cream in a microwave-safe bowl, and heat till the cream is showing bubbles, and the caramel is soft. Stir the caramel and cream till smooth. (I did this in 3-30 second intervals)
Spoon the caramel over the cake, dripping it down into the crevices around the apples.
Scatter the walnuts on top. Sprinkle lightly with very coarse sea salt flakes if desired.