Start by preheating your oven to 400 degrees Fahrenheit while you get your donut pan ready. Lightly grease every well with nonstick spray so the donuts release cleanly once baked. Set the pan aside on your counter while you mix the batter.
In a large mixing bowl, add the dry cake mix and the entire can of pumpkin puree. Using a hand mixer or a sturdy whisk, blend the two together until the batter is completely smooth with no dry streaks left. Stir in the pumpkin pie spice here if you're using it.
Transfer your batter into a piping bag or a zip-top bag with the corner snipped off. Pipe the batter evenly into each donut well, filling them about two-thirds full so they have room to rise without overflowing.
Slide the pan into your preheated oven and bake for 12 to 15 minutes, until the donuts spring back lightly when touched and no longer look wet on top. Every oven runs a little differently, so start checking around the 12-minute mark.
Let the donuts rest in the pan for about 5 minutes before transferring them to a cooling rack. Once they've cooled slightly, whisk together your powdered sugar, milk, and cinnamon, then dip or drizzle each donut before serving.