Preheat oven to 350 degrees F. Lightly spray a 12" pizza pan (use one with a solid bottom please) with non stick spray.
Open up the can of pumpkin puree and remove 2 tbsp and set aside for later use. Combine the remaining pumpkin puree with the brownie mix (just the powdered mix) and use a sturdy wooden spoon to stir. Once the mixture is thick and wet throughout pour onto the pizza pan and spread out to an even thickness. Bake for 20 to 25 minutes or until a toothpick inserted in the center comes out clean. Let cool in pan on wire rack.
To Make The Frosting:
Place the 1/2 cup butter in your mixing bowl fitted with the paddle attachment and beat until light and fluffy. Add the powdered sugar, vanilla and pumpkin spice to the bowl and beat slowly until the powdered sugar becomes moist, then increase speed and beat at medium speed for 1 to 2 minutes, while gradually adding in the 2 tbsp of pumpkin puree and 1 tbsp of heavy cream. Beat until fluffy.
Spread on top of the cooled brownie pizza and top with any additional toppings you like. Enjoy!