In a large mixing bowl, cream together butter and brown sugar. Add pumpkin puree and molasses and mix until creamy.
In a separate large mixing bowl, combine flour, pumpkin pie spice, baking soda, baking powder, and salt.
Gradually add flour to butter mixture and mix until completely combined. The dough will be stiff.
Flatten dough out into a round circle and wrap in plastic wrap.
Refrigerate at least 30 minutes.
When ready to bake, preheat oven to 350 degrees.
Remove cookie dough from plastic wrap and place between two pieces of parchment paper.
Roll dough out until it is approximately 12" in diameter.
Remove top layer of parchment paper and place cookie pizza with bottom layer of parchment paper on top of a large cookie sheet or pizza pan.
Bake for 20-25 minutes or until cookie is slightly puffy and browned.
Remove from oven and cool completely.
To make frosting, cream together butter with 1 cup of powdered sugar, vanilla extract, and 1 tablespoon of heavy cream. Add another cup of powdered sugar and 1 tablespoon of heavy cream. Mix until well combined. Add last cup of powdered sugar and 1 tablespoon of heavy cream. Mix well. If frosting is stiff, add extra tablespoon of heavy cream.
Frost and decorate cookie as desired.
Store in an airtight container in refrigerator for up to one week (if it lasts that long!)
Nutrition information calculated using myfitnesspal is provided as a courtesy, but will vary depending on the specific brands of ingredients you use. Please consult with your doctor regarding specific health needs.