Preheat oven to 350 degrees. Line 2 (12 cup) muffin tins with cupcake liners. Combine cake mix, pumpkin, and water in a large bowl, and mix until batter is smooth. Bake cupcakes for 20-25 minutes, or until a toothpick comes out clean.
Meanwhile, to make the frosting, combine milk and pudding mix in a medium size bowl. Whisk until pudding mix and milk are well combined, and mixture is smooth. Fold in the cool whip.
To prepare the frosting to pipe on the cupcakes, take a large ziplock back and turn it ‘inside out’. Using a spatula, scoop the frosting into one of the corners of the bag. Then turn bag ‘right side out’, and push frosting all the way down into the corner of the bag. Set frosting bag in the fridge until you are ready to frost the cupcakes.
Once cupcakes are completely cooled, you are ready to begin frosting! Take the frosting bag out of the fridge, snip the corner of the bag off with sharp scissors. To frost the cupcakes, squeeze the frosting out of the bag while making a swirling/upward motion and lift at the peak! Garnish with sprinkles or cinnamon if desired! Serve immediately, or keep in fridge to keep frosting chilled until serving!