1/3 cup unsalted butter, melted, plus 1/4 cup, room temperature
3/4 cup heavy cream
2/3 cup unsweetened cocoa powder
1 3/4 cup confectioners’ sugar, plus more for serving
Preheat oven to 350 degrees, with racks in upper and lower thirds. In a medium bowl, whisk together flour, pumpkin pie spice, baking powder, and salt. In a large bowl, whisk together eggs, pumpkin puree, brown sugar, melted butter, and 1/4 cup heavy cream. Add flour mixture to egg mixture and whisk just until combined.
Drop dough in 2-tablespoon mounds, 1 inch apart, onto two parchment-lined baking sheets. Bake until tops of cookies spring back when gently pressed, about 14 minutes, rotating sheets halfway through. Transfer to wire racks and let cool completely.
In a medium bowl, using an electric mixer, beat room temperature butter, 1/2 cup cream, cocoa powder, and confectioners’ sugar on medium-high until fluffy, 2 minutes. Spread filling on flat side of half the cookies; sandwich with remaining cookies. To serve, dust with confectioners’ sugar.