Preheat the oven to 350 degrees Fahrenheit and prepare a 9x13'' baking dish with cooking spray.
In a large mixing bowl add the dry ingredients for the cake batter: the spice cake mix and pumpkin pie spice. Stir to combine.
Add the wet ingredients: pumpkin puree and eggs.
Mix with a spoon until fully combined. I do not recommend using a mixer, as it will cause the cake to be dense, not fluffy.
Transfer the cake mix to the prepared pan and smooth using a rubber spatula.
Bake for 25-28 minutes or until a toothpick in the center of the cake comes out clean.
Place the unfrosted cake on a wire rack. Let the cake cool for at least 60 minutes before frosting.
Make the frosting in a large bowl. Add the cream cheese, butter, vanilla and cinnamon. With a hand mixer or stand mixer, beat the ingredients for about 30 seconds, until well combined.
Slowly add in the powdered sugar, mixing for 6-8 minutes. I know this is a long time, but it will give you super light and fluffy frosting!
Once the cake is completely cooled transfer the frosting to the cake. With a spatula, evenly spread the frosting over the cooled cake. Once frosted, sprinkle the top with the walnuts and any other toppings you'd like to add.
Notes
If you'd like a stiffer frosting, add up to 1 more cup of powdered sugar.Add white chocolate chips or cinnamon chips to the batter for some extra flavor!If making pumpkin cupcakes, this recipe will make 12 large cupcakes. You will need to bake for around 15 minutes.Make your own homemade pumpkin pie spice by combining 3/4 teaspoon of cinnamon with a pinch each of allspice, ginger, ground cloves and nutmeg.