1. Preheat oven to 350ºF. Line 2 rimmed baking sheets with parchment paper or silicone baking mats.
2. In a large bowl combine the brown sugar, oil, eggs, pumpkin puree and 1 teaspoon vanilla extract. Beat well.
3. Fold in the flour, salt, baking powder, baking soda and 1 tablespoon Pumpkin Pie Spice. Mix until just combined.
4. Spoon 1/4 cup mounds of batter about 2 inches apart onto the prepared pans. Bake for 13-15 minutes or until cakes spring back when touched. Cool completely.
To Make Filling:
1. Combine the cream cheese and butter in the bowl of a stand mixer fitted with paddle attachment or with a hand-held mixer. Beat until light and fluffy, about 4 minutes.
2. Add in confectioner's sugar. Beat until smooth, about 3 minutes. Stir in remaining 1/2 teaspoon Pumpkin Pie Spice and vanilla extract.
1. Spread 2 tablespoons of filling on flat sides of half of the cakes and top with the remaining cakes.
The whoopie pies will keep for up to 3 days wrapped in plastic wrap. Keep refrigerated.