To make the cake layer, start by preheating the oven to 350 degrees. Spray a 13×9 inch baking dish with non-stick spray. Combine cake mix, pumpkin puree, and water in a large bowl. Mix until well combined, and transfer into the baking dish. Bake for 25-30 minutes, or until a toothpick comes out clean. When completely cooled, crumble the cake (or cut into small squares depending on the look you want) and set aside.
While cake is baking, in a large bowl, whisk milk and pudding mix together for about 2 minutes or until slightly thickened. Let stand for 2 minutes or until soft-set. Fold in 1 container of cool whip, and 1/2 cup of pumpkin, brown sugar, and spices until well blended. (Note: Leave the other whipped topping as is for the whipped cream layer).
In a trifle bowl or (a large pretty glass serving bowl), layer half of the cake, half of the pudding/pumpkin mixture, and half of the whipped topping. Repeat layers until the trifle glass is filled to the top. (I like to end with the whip cream layer!)
Garnish with cinnamon or crushed gingersnaps if desired, (you may also layer the gingersnaps throughout the layers. Refrigerate for 1 hour, or until ready to serve. (Also tastes great when made the night before!)
Note: You may also use wineglasses or individual dessert glasses for individual trifles!*