Preheat oven to 350 degrees. Spray a 9-inch round cake pan with cooking spray and set aside.
In the bowl of a stand mixer, whip together the butter and 1 cup of the sugar until pale and fluffy, 2 to 3 minutes.
Add the eggs and vanilla and beat until smooth.
In a medium bowl, whisk together the flour, baking powder and salt. Gradually add the flour mixture to the butter mixture alternately with sour cream, beginning and ending with the flour mixture, beating just until combined after each addition.
Fold in the apples. Spread the batter in the prepared cake pan
Sprinkle the walnuts evenly on top of the cake batter.
Sprinkle the remaining 1/4 cup of sugar evenly over the walnuts.
Bake until a wooden pick inserted in the center comes out clean, 35-40 minutes.
Let the cake cool in the pan and slice and serve the cake from the pan. This cake is even better if it is still warm when served!
Drizzle each slice of cake with salted caramel sauce to serve.