Preheat the oven to 350°F and line 2 baking sheets with parchment paper.
In a mixing bowl, whisk together flour, baking powder, baking soda, cinnamon, salt, and pumpkin pie spice. Set aside.
In a large mixing bowl, using a hand mixer, beat brown sugar, oil, pumpkin puree, and vanilla until well combined on low speed, about 30 seconds.
Add eggs one at a time and mix until combined.
Add the dry ingredients to the wet ingredients and mix on low until just combined. Use a rubber spatula to scrape the bottom and sides of the bowl and give a final stir until no streaks of flour remain. The batter will resemble a thick cake batter instead of a stiff cookie dough.
Scoop 2 tablespoons of the whoopie pie batter onto the baking sheet 2” apart.
Bake 10-15 minutes until the cakes are puffed, the tops are dry, and a toothpick inserted comes out clean.
Cool sandwich cookies on the baking sheet for 4-5 minutes, then transfer to a wire rack until they are cooled. Try not to let them sit on the hot pan too long or the bottoms will get too soft to move. While they cool, make the filling.