1. Heat oven to 325 degrees . Coat 12 cups of a jumbo muffin tin with nonstick cooking spray. (Note: Cake can alternately be baked in a 15 x 10 x 1-inch jelly roll pan.)
2. Prepare cake mix as per package directions with oil, eggs and 1 cup water. Fold in grated apple and cinnamon. Divide evenly among prepared muffin cups. Bake at 325 degrees for 25 to 28 minutes, until crowned slightly and dry to the touch.
3. Cool in pans for 10 minutes, then run a thin knife or spatula around edge and remove cakes directly to a wire rack; cool completely.Filling
4. Melt butter in a large nonstick skillet over medium heat. Add salt, diced apples and cinnamon. Cook 7 to 8 minutes, until slightly tender. Set aside.
5. Whip heavy cream and sugar to medium-stiff peaks.
6. Once cakes have cooled, slice each in half horizontally. Fit 1 cake round into a jar, pressing with fingers until flush with bottom of jar. Top with 1 tbsp of the caramel, 1 tbsp of the apple mixture and 2 tbsp of the whipped cream. Repeat layers of cake, 1 tbsp of the caramel and 1 tbsp of the apple mixture. Repeat process in remaining jars. Refrigerate, then top with a dollop of whipped cream and drizzle with a little of the remaining caramel just before serving.