Preheat oven to 350 degrees. Line an 18x13 lightly colored sheet pan {with sides!} with parchment paper. Spray with nonstick cooking spray and set aside. In a large bowl, mix cake mix, pumpkin, eggs, water, oil and pumpkin pie spice together until combined.
Batter should be thick and mostly lump free.
Spread evenly onto the prepared sheet pan and bake for 13-17 minutes or until a toothpick comes out clean from the center. Remove and set aside to cool.
While the cake is baking, stir cream cheese and sugar together until smooth.
Stir in pumpkin, vanilla, milk, cream, and cinnamon until thick and fluffy. Scrape sides and mix again briefly to ensure a smooth topping.
As the pumpkin mixture sits, it should thicken. Refrigerate if cake isn't cooled completely.
Once cake is completely cooled, spread evenly over entire cake and refrigerate while you make the whipped cream.
For the cream, whip heavy whipping cream, vanilla, and powdered sugar together until stiff peaks form. Pipe large rosettes onto chilled cake and sprinkle with more pumpkin pie spice. Refrigerate until ready to serve. Cut into 24 square pieces.