Preheat oven to 350°F. Grease a 9x13 baking dish and set aside.
In a small bowl, mix brown sugar and cinnamon. Set aside.
In a large bowl, whisk together cake mix, eggs, sour cream, butter, vegetable oil, pumpkin puree, water extract and pumpkin pie spice until smooth.
Pour half of the batter into the prepared pan. Sprinkle the brown sugar mixture over the batter, then dollop the remaining batter on top. Using a knife, insert it half ways into the batter and swirl back and forth along the cake.
Bake for 40-50 minutes, or until lightly golden brown (a toothpick inserted in the middle should come out clean).
While cake is baking, make icing in a medium bowl by whisking together the sifted powdered sugar, half and half and cinnamon until smooth.
Pour the icing over the cake while warm and allow to cool before cutting and serving.