Preheat oven to 350 degrees F and grease a glass 9x13 baking pan with nonstick cooking spray. Set aside.

Add cake mix, pumpkin puree and milk together and mix well. Spread batter evenly into prepared pan and bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. Remove from oven and let stand to cool for 15 minutes. Poke holes into the cake with a fork.

Whisk together sweetened condensed milk and pumpkin pie spice and gently pour on top of the cake, making sure you pour it into the fork holes. Let stand to cool completely before covering and refrigerating until ready to serve.

When ready to serve, top with whipped topping and slice.