In a large bowl, mix together the flour, baking powder, baking soda, and salt.
With an electric mixer or whisk, combine pumpkin puree, sugar, oil, eggs and vanilla until smooth.
Slowly add the dry mixture to the pumpkin mixture, mixing until a cake batter forms and ingredients are just combined.
Transfer the mixture to a 13x9-inch baking dish. Bake for 30-35 minutes, or until it's golden brown, and a toothpick inserted into the center comes out clean.
While your cake is baking, make your milk mixture. Whisk together your whole milk, sweet condensed milk, evaporated milk, pumpkin pie seasoning and liquor, if using.
Once your cake is cooked, allow it to cool completely. Then, poke holes throughout your cake using a fork. I like to poke about 20-25 holes. Pour the milk mixture all over the cake. You may need to move the cake pan around to absorb the milk as you continue to pour.
Cover your cake and chill in the refrigerator for at least 6 hours for overnight for best results.
Make the topping before serving. Using an electric mixer, whisk together heavy cream until it is creamy, then slowly add in powdered sugar, until it reaches a whipped cream consistency. Spread over the top of the cake and lightly sprinkle with cinnamon, if desired. Slice and serve!
Notes
Storage: Store leftovers covered in the fridge for up to 3 days.