Line 2 baking sheets with parchment and lightly coat with cooking spray.
Whisk together the flour, baking soda, baking powder and salt in a medium bowl. Beat together the granulated sugar, butter and vanilla in a large bowl with an electric mixer on medium-high speed 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 in 2 batches, beating after each addition until just combined.
Scoop 2-tablespoon mounds of batter onto the baking sheets about 2-inches apart. Moisten your fingers and gently form each mound into a round and slightly smooth the top. Refrigerate until firm, about 30 minutes.
Preheat the oven to 400 degrees F.
Bake the cookies, rotating the trays halfway through, until the tops bounce back when gently touched, 10 to 12 minutes. Cool on the baking sheets 5 minutes, then transfer to a rack to cool completely.
For the buttercream filling:
Put the confectioners' sugar and butter in the bowl of an electric mixer fitted with the paddle attachment and beat on low speed until just incorporated, about 1 minute. Increase the mixer speed to medium and beat until smooth and fully incorporated. Turn the mixer off and scrape down the sides of the bowl. Add the dissolved cocoa powder and vanilla extract. Turn the mixer back on to medium speed and beat until everything is well mixed, 1 to 2 minutes. With the mixer off, add the milk. Mix together on low speed until just incorporated.
To assemble the whoopie pies: Flip half of the cookies over and spread the bottoms with the filling. Top each with another cookie--bottom facing in--to make a sandwich.
Put the sprinkles on a plate and roll the edge of each whoopie pie in the sprinkles to coat.
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