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Preheat the oven to 350 degrees and line two large baking sheets with parchment paper or Silpat liners. In a small bowl, combine the flour, pumpkin pie spice, baking powder, soda and salt.
Cream the butter and sugar until fluffy, then add the eggs and oil, mixing until smooth. Beat in the sweet potato. (The mixture may separate—that’s okay.) Add half the flour, then the buttermilk, then the remaining flour, mixing well each time.
Drop the whoopie pies by teaspoon onto the prepared baking sheet, leaving about 1 inch between each spoonful. Ideally, you can fit 15 pies on each sheet.
Bake for 8 to 10 minutes, until dry and just browning around the edges but not browned on top. They should remain tender and soft.
Cool the cookies completely, then spread a dollop of marshmallow fluff on the flat side of half the cookies, topping with a second half. Serve the same day.