For buttercream filling
3 tablespoons unsalted butter, at room temperature
1 cup plus 2 tablespoons powdered sugar
1 tablespoon strongly brewed coffee, plus extra if filling is too thick
To prepare cookies:
In a medium bowl, whisk together flour, espresso powder and salt, and reserve.
In a bowl of an electric mixer on medium-high speed, beat butter until creamy, about 1 minute. Gradually add granulated sugar and beat until light and fluffy, about 2 minutes. Add egg and beat until thoroughly combined, scraping down sides of bowl as necessary. Add vanilla extract and beat until thoroughly combined. Reduce speed to low and gradually add cocoa powder, scraping down sides of bowl as necessary and mixing until just incorporated. Add flour mixture in 3 additions, mixing until just incorporated. Divide dough into two equal pieces, shape into disks, wrap in plastic wrap and refrigerate at least 2 hours or overnight.
When ready to bake, preheat oven to 325 degrees and line baking sheets with parchment paper. On a lightly floured work surface, using a lightly floured rolling pin, roll dough to 1/4-inch thickness. Using a 2-inch round or scalloped cookie cutter, quickly cut as many circles as possible and place cookies 1 inch apart on prepared baking sheets. Repeat with remaining dough, gathering up scraps, re-rolling and cutting until all dough is used. Bake cookies, 1 sheet at a time, until they no longer look wet and are soft to the touch, 7 to 9 minutes; do not overbake. Remove from oven and cool 2 minutes before transferring cookies to a wire rack to cool completely.
To prepare buttercream filling:
In a bowl of an electric mixer on medium-high speed, beat butter until creamy, about 1 minute. Reduce speed to low and gradually add powdered sugar, scraping down sides of bowl as necessary. Add coffee and mix until blended (adding more, 1 teaspoon at a time, if necessary to reach desired consistency). Increase speed to high and beat until buttercream is creamy, about 2 minutes.
To assemble cookies: Spoon 1 teaspoon buttercream filling into center of bottom of a cookie. Top with another cookie, bottom side up as if making a sandwich, and gently press cookies together until filling spreads evenly to about 1/16-inch from edge. Repeat until all cookies form sandwiches.
2009 Finalist: Jane Stern of Minneapolis, Minnesota, who adapted recipe from “The Good Cookie,” by Tish Boyle