Preheat the oven to 350℉. Spray a 9x13 baking dish with cooking spray. Set aside.
Yellow Cake Layer : In a mixing bowl, add the yellow cake mix, large eggs, milk, and oil. Whisk together until the cake is batter is smooth and combined well.
1 box (15.25 oz) yellow cake mix, 3 large eggs, 1 cup whole milk, 1/2 cup canola or vegetable oil
Pour the cake batter into the prepared pan making sure to spread it out evenly to the edges to create an even layer.
Pumpkin Pie Layer : In a separate mixing bowl, whisk together the pure pumpkin, heavy whipping cream, pumpkin pie spice, and light brown sugar until combined.
1 can (15 oz) pure pumpkin, 1 cup heavy whipping cream, 2 teaspoons pumpkin pie spice, 1/2 cup light brown sugar
Add the eggs, and continue whisking just until the eggs are incorporated. Do not over-mix the eggs.
3 large eggs
Spoon the pumpkin pie mixture over top the cake batter layer inside the pan. Make sure to cover the cake layer entirely (as best you can) with the pumpkin pie batter. * It's important to spoon the pumpkin batter over top the cake batter to ensure that you get an even layer of it. If it's dumped onto the center then you won't get that nice layered look once the cake is baked. Bake the cake for 50-55 minutes or until the top of the cake is golden brown and the pumpkin layer has mostly sunken to the bottom of the pan. A toothpick inserted into the center should come out clean (minus the custard-like pumpkin layer).
Remove the cake from the oven and set it on a cooling rack. Let the cake cool completely for 1 hour.
Pumpkin Spice Pudding Frosting : Add the vanilla instant pudding mix and whole milk to a mixing bowl. Whisk together for 2-3 minutes or until the pudding starts to thicken. Let the pudding sit for another 2 minutes. Fold in the thawed Cool Whip.
1 box (3.4 oz) instant vanilla pudding mix, 2 cups whole milk, 1 teaspoon pumpkin pie spice, 8 ounces Cool Whip
Spread the pudding frosting over top of the cooked cake. Cover the cake with a lid or plastic wrap, and refrigerate for at least 4-6 hours before serving.
Cut the cake into squares and serve chilled.