For the Pumpkin Whoopie Pies

1. Preheat your oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper. This is extremely important, as the whoopie pies will stick to the pan if you do not line or spray the pan properly.
2. In a medium bowl, whisk all purpose flour, baking powder, baking soda, salt, cinnamon, nutmeg, and ginger together until combined.
3. In a large bowl, whisk vegetable oil, room temperature eggs, vanilla extract, packed light brown sugar, granulated sugar, and pumpkin puree together until smooth.
4. Carefully, add the dry ingredients to the wet, using a whisk to mix the batter together. Only mix until you see all of the dry ingredients disappear into the batter. Stir just a few more times until smooth, as mixing past this point will dry out your whoopie pie disks.
5. Let the batter sit for 5 minutes, as this step is critical in slightly thickening the batter. This helps the whoopie pies rise rather than spreading as they bake.
6. Use a tablespoon to measure the batter into 1-1/2 tablespoon size dollops. Place the circles of batter at least 2 inches apart, as they will spread and rise.
7. Bake the whoopie pies for 7-11 minutes. The tops should spring back when slightly poked in the center, and a toothpick inserted into the center of one will come out with just a few moist crumbs. The tops should look shiny in color and should not look wet in the center.
8. Let the whoopie pies cool on the pan, but run a spatula or knife under each whoopie pie after about 5 minutes of cooling to ensure they do not stick to the pan or paper.

For the Marshmallow Filling + Assembly

1. In a large bowl with a hand mixer or in the bowl of your stand mixer fitted with the paddle attachment, beat butter on medium speed until smooth.
2. Slowly, add the powdered sugar to the butter while mixing on low speed. Use a spatula to scrape down the sides of the bowl as needed.
3. Add marshmallow fluff to the mixture on medium speed and beat until smooth. Pour the heavy whipping cream into the frosting and beat on medium high speed for one minute until smooth and velvety.
4. If desired, transfer the filling to a piping bag with a round tip or large Ziploc bag with the corner of the bag cut off. This will make the filling of the whoopie pie sandwiches easier, but you are more than welcome to use a spoon or knife.
5. Top half of the disks with marshmallow filling on the flat side. Then, top each with another pumpkin cake disk with the flat side down.
6. Individually wrap each whoopie pie in cling wrap to ensure they do not stick to one another when storing. Store the whoopie pies at room temperature or in the fridge for up to 4 days. Whoopie pies can be frozen, individually wrapped, for up to 3 months after assembly.