Line 2 baking sheets with parchment and lightly coat with nonstick spray.
Whisk together the flour, cocoa powder, espresso powder, baking soda and salt in a medium bowl. Beat the brown sugar, butter and vanilla together 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 into a round and slightly smooth the top of each mound. Refrigerate until firm, about 30 minutes.
Preheat the oven to 400 degrees F.
Bake until the tops of the cookies bounce back when gently touched, rotating the trays halfway through, 10 to 12 minutes. Cool on the baking sheets 5 minutes, then transfer to a rack to cool completely.
For the buttercream filling:
Beat the butter in a large bowl with an electric mixer on medium speed until fluffy. Gradually add half of the confectioners' sugar, beating. Beat in the milk 1 tablespoon at a time, then add the remaining sugar and beat until incorporated, smooth and creamy, scraping the bowl as needed, about 3 minutes. Mix in the vanilla. Beat in the dulce de leche until smooth.
To assemble the whoopie pies:
Flip half of the cookies over and spread the bottoms with the filling. Top each with another cookie--bottoms facing in--to make a sandwich.
Yield: 12 whoopie pies
Active Time: 40 minutes
Total Time: 2 hours (includes chilling and cooling times)
Ease of preparation: easy
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