Preheat oven to 350°F. Coat a standard 12-cup Bundt pan with nonstick baking spray.
In a large mixing bowl, whisk together flour, baking soda, salt, cinnamon, and ground walnuts.
In the bowl of a stand mixer fitted with paddle attachment, beat together oil and sugar until well mixed, about 1 minute on medium speed.
Add the eggs, vanilla bean paste, and yogurt. Continue mixing on medium for 2 minutes, scraping the sides halfway through.
Gently fold wet ingredients into dry ingredients until only a few streaks of flour are remaining.
Add grated apple, then fold until just incorporated.
Pour batter into prepared Bundt pan until about ⅔ full. Pour caramel sauce evenly on top, then swirl into cake batter using a butter knife.
Top with remaining batter, smoothing the top with a spatula.
Bake for 1 hour, or until a cake tester inserted into the cake comes out with only a few moist crumbs.
Let cake cool for 10 minutes before inverting onto a wire rack. Enjoy warm or at room temperature. Serve with a light dusting of confectioner’s sugar (optional).