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, 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. You want a good number of large holes so that there is plenty of room for the eggnog pudding to sink into the cake.
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