Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

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

Add pumpkin puree, eggs, and vanilla extract. Mix on low speed for 1-2 minutes until smooth.

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

Fold dry ingredients into wet mixture gently to combine without overmixing.

Using a cookie scoop, dollop spoonfuls of dough onto lined baking sheets, spaced 2 inches apart.

Bake for 11-13 minutes until edges are lightly browned. Let cookies cool for a few minutes before transferring to a wire rack.

While cookies cool, prepare the buttercream by beating softened butter until creamy, then gradually add powdered sugar and salt.

Incorporate heavy cream and mix until light and fluffy (about 2 minutes).

Assemble whoopie pies by spreading buttercream on flat side of a cookie and topping with another cookie.

Optionally roll edges in sprinkles or chopped nuts for decoration.