Preheat oven to 350 degrees. Prepare cake mix according to package instructions. Pour into a sprayed 9x13' cake pan. DO NOT BAKE! Set aside.
In another bowl, whisk together the canned pumpkin, evaporated milk, heavy cream, eggs, brown sugar, and pumpkin pie spice until smooth. Slowly pour the pumpkin pie mixture all over the top of the cake mix.
Carefully place the cake into the oven and bake for 50-60 minutes or until center is no longer jiggly, and a toothpick inserted in the cake comes out clean. I baked mine for 60 minutes. NOTE: The cake may be slightly darker than normal, but still tastes great! Let cool to room temperature.
Prepare your frosting. Place pudding into large bowl. Add the pumpkin pie spice and pour in the cold milk. Whisk or beat until combined and the pudding starts to thicken. Gently fold in the cool whip until its completely combined. Spread on top of the cooled cake. You can serve now or chill the cake and serve when chilled.