For the cookies: Line 2 baking sheets with parchment paper and lightly coat with nonstick spray.
Whisk together the flour, cocoa powder, baking soda and salt in a medium bowl. Beat the brown sugar, butter and vanilla with an electric mixer on medium-high speed in a large bowl until light and fluffy. Add the egg and beat until combined. Reduce the speed to low and add the flour mixture in 3 batches, alternating with the buttermilk and beginning and ending with the flour mixture, until just combined.
Scoop 1-tablespoon mounds of batter on the baking sheets about 1 inch apart. Moisten your fingers, gently form each mound into a round and slightly smooth the top of each. Refrigerate until firm, about 30 minutes.
Adjust oven racks to the top and bottom thirds of the oven and preheat to 400 degrees F.
Bake until the tops of the cookies bounce back when gently touched, rotating the baking sheets halfway through, 10 to 12 minutes. Cool on the sheets 5 minutes, then transfer to a wire rack to cool completely.
For the filling: Beat the cream cheese and butter with an electric mixer on medium-high speed in a medium bowl until smooth and creamy. Add the pumpkin puree and beat to combine. Add the confectioners' sugar, pumpkin pie spice and salt and beat to combine.
Scoop 1 tablespoon of the filling on the bottoms of half the cookies. Sandwich with the remaining cookies and press gently to push the filling to the edges. Refrigerate to chill before serving, about 1 hour.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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