Preheat oven to 350 degrees. Gather two nonstick baking sheets.
In a small bowl, whisk together flour, baking powder & soda, salt, cinnamon, ginger, and nutmeg.
In a medium bowl, add oil, brown sugar, and eggs. Beat with a hand mixer for 1 minute. Add pumpkin. Beat again for 1 minute.
Fold flour mixture into pumpkin mixture with a large spoon until well incorporated and all the flour is mixed in.
Place approx. 2 Tbsp of dough 1 inch apart on the baking sheets. The cookies don't spread too much so make them about the size and shape you want.
Bake for 13-15 minutes or until cookies are firm in the center.
While cookies are baking, prepare the filling. In a new bowl, combine all the filling ingredients and mix with hand mixer until fluffy.
Scoop filling into the corner of a freezer bag and keep in refrigerator until you're ready to assemble the whoopie pies.
When your cookies are done baking, allow to cool on the baking sheet for 5-10 minutes before removing. Allow to completely cool on a baking rack before filling.
To assemble the whoopie pies, cut a hole in the end of the freezer bag filled with filling. Pipe onto one cookie. Top with another equally sized cookies. Repeat for all 8 cookies.
Store any extra in the refrigerator.