These whoopie pies may be small, but they pack big flavor. Intense chocolate filling enhances the cakey pumpkin cookies.
Make the cookies: Preheat oven to 350 degrees. Line 2 large baking sheets with parchment or a nonstick baking mat.
Whisk together flour, baking powder, baking soda, salt, and spices in a bowl. In another bowl, whisk together sugar and oil until well combined. Add pumpkin puree, and whisk until combined. Whisk in eggs and vanilla. Add flour mixture to pumpkin mixture; whisk until fully incorporated.
Drop heaping tablespoons of dough onto prepared baking sheets, about 1 inch apart. Smooth tops with a wet fingertip. Bake, rotating sheets halfway through, until cookies just start to crack on top and a cake tester comes out clean, about 15 minutes. Transfer sheets to wire racks, and let cookies cool completely.
Make the filling: With an electric mixer on medium-high, beat butter, cream, cocoa, and confectioners' sugar until pale and fluffy, 2 to 3 minutes.
Spread filling (about 1 scant tablespoon) on flat side of half the cookies; sandwich with remaining cookies, pressing to adhere. (Whoopie pies are best eaten the same day but can be refrigerated in airtight containers overnight.)
A small ice-cream scoop with a release mechanism makes it easy to form uniform-sized cookies.