Preheat oven to 350°F and spray a 9x13-inch baking pan with nonstick spray. Set aside.
Whisk the white cake mix and pumpkin pie spice together in a large mixing bowl.
15.25 ounces white cake mix, 2 teaspoons pumpkin pie spice
Using a hand mixer or a whisk, beat in the eggs, milk, and oil until the batter is completely combined and smooth, about 2 minutes.
3 large eggs, 1 cup milk, ½ cup vegetable oil
Pour the cake batter into the prepared cake pan and bake in the oven for 34-38 minutes, or until a toothpick comes out clean.
Let the cake cool for 5 minutes before using the end of a wooden spoon or a cupcake corer to poke holes throughout the entire top of the cake, about 1 inch apart.
Prepare the pudding by mixing the instant vanilla pudding mix with the 3 cups of eggnog in a large bowl.
6.8 ounces instant vanilla pudding mix, 3 cups eggnog
Immediately pour the pudding mixture over the entire cake, being sure to fill all of the poked holes. Use a spatula to spread the pudding evenly over the cake.
Allow the cake to cool for an additional 10-15 minutes and then cover with plastic wrap and chill in the refrigerator for 1-2 hours.
When ready to serve, spread the whipped topping evenly over the cake and sprinkle with cinnamon, optional.
8 ounces whipped topping, Ground cinnamon