Preheat oven to 350 degrees
Empty cake mix and pumpkin puree into a large bowl using a mixer beat until well incorporated. The batter will be very thick, but will come together nicely. Pour batter into a 7-by-11-by-2-inch baking pan. Bake for 28 minutes or until toothpick inserted into center comes out clean. Do not overbake.
Let cool for 5-10 minutes in the pan, then flip onto a platter.
While you cake is cooling, combine powdered sugar, apple cider vinegar and pumpkin pie spice. Glaze should be thick but pourable. Add more sugar or vinegar if needed. Pour over the cake while still warm.
Reserve some to pour over each slice when served.