Preheat oven to 350F and spray a bundt pan (I used a 12-cup pan) with nonstick spray. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment (or in a large mixing bowl with an electric hand mixer), combine spice cake mix, oil, applesauce, apple cider, cinnamon, eggs, vanilla extract, and flour. Mix on low speed for about 30 seconds, then scrape down the sides of the bowl. Continue mixing on medium-high speed for about 2 minutes
Pour batter into prepared bundt pan. Bake for about 35-40 minutes, or until a toothpick inserted in the center comes out clean.
Remove from oven and let cake cool in the pan for about 15 minutes, then run a knife along the edge of the cake to ensure it's separated from the pan. Invert the cake onto a wire rack to cool completely.
For easy clean-up, place a sheet of parchment paper under the wire rack before adding topping to the cake. Then brush the outside surface of the cake with melted butter, then let it sit for a couple of minutes.
Prepare the topping by whisking together sugar and cinnamon in a small bowl. Sprinkle the cinnamon sugar over the melted butter, pressing it into the sides and top of the cake, covering the whole surface of the cake (I use my hands for this).
Transfer the cake to a serving plate. Slice and enjoy!
Notes
If needed, you can omit the oil and use a total of 1/2 cup of applesauce, or you can omit the applesauce and use a total of 1/2 cup of oil. I used regular sweetened applesauce, but you could also use cinnamon applesauce.
Make sure to use apple cider, NOT apple cider vinegar or plain apple juice.
Leftover cake can be stored, covered, at room temperature for up to 3-4 days.
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