Preheat the oven to 350 degrees.
Line two cookie sheets with parchment paper.
Whisk together the flour, pumpkin pie spice, baking powder, and baking soda together in a large bowl.
In a different bowl, mix together the brown sugar, sour cream, and 1/2 cup of butter until thoroughly combined. I used a stand mixer for this step.
Mix in the pumpkin puree.
Add the eggs one at a time, beating between each.
Mix in the 1 tsp of vanilla.
Stir the pumpkin mixture into the flour mixture. I did this by hand. Mix until thoroughly combined.
Using a cookie scoop, drop a large circle of the batter onto the cookie sheet. Try to keep an evenly round shape. Leave about 2 inches between cookies.
Bake for 15 minutes (mine took 16). My oven is small and I am impatient, so I baked two trays at once, putting the top pan on the lower rack and the lower on the top rack, midway through the bake time. The whoopie pies are ready when they are springy to the touch and lightly browned on the bottom.
Let the pies cool and prepare the cream cheese frosting.
Beat the cream cheese and 1/2 cup of butter together until smooth.
Add in 2 tsp of vanilla.
Add in the sugar, one cup at a time. You may find you need a little more get your desired texture.
Once the whoopie pies are completely cool, spread on some frosting. I used a pastry bag, but you can use a knife or a spoon.
Cover with another whoopie pie to make a "sandwich."