Preheat your oven to 350°F (175°C) and prepare your baking sheet by lightly greasing it or lining it with parchment paper.

In a large mixing bowl, combine vegetable oil and brown sugar, blending on medium speed until light and fluffy, about 2-3 minutes.

Add pumpkin puree, eggs, and vanilla extract to the creamed mixture, beating on medium speed until well combined, about 1-2 minutes.

In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and pumpkin pie spice.

Gradually add the dry mixture to the wet mixture, stirring gently until just combined.

Drop rounded portions of dough onto the baking tray, spacing them about 2 inches apart, and bake for 11-13 minutes.

Allow the cookies to cool on the baking sheet for about 5 minutes before transferring to a wire rack.

In another mixing bowl, beat softened butter until creamy, then gradually add powdered sugar, salt, pumpkin pie spice, and heavy cream until fluffy.

Spread a generous amount of buttercream on one cookie and top with another, creating a sandwich.

Store your whoopie pies in an airtight container at room temperature for up to 4 days.