12tablespoonsunsalted buttersoftened (6oz or 170g)
For the chocolate filling:
¼cupunsweetened cocoa powder(23g)
6tablespoonsunsalted buttersoftened (3oz or 85g)
1tablespoonlight corn syrup
To prepare the cookie dough:
In a small mixing bowl, whisk together the flour, cocoa and salt. Set aside.
In a large mixing bowl, beat together the butter and powdered sugar until light and fluffy. Add the egg and mix just until blended. In a small measuring cup, combine the vanilla and espresso powder. Stir until the espresso is dissolved. Add the vanilla to the butter and sugar mixture. Beat on medium speed until blended. Scrape the bottom and sides of the bowl with a spatula; mix again until incorporated.
Add half the flour mixture and blend on low until incorporated. Blend in the remaining flour and cocoa and beat until blended. Refrigerate for 30 minutes or up to one hour.
When ready to bake, preheat oven to 350°F. Line two baking sheets with parchment paper.
Remove the dough from the refrigerator and allow to sit at room temperature for 5 - 10 minutes or until pliable. On a large piece of parchment paper or wax paper, roll a portion of the dough out to 1/4-inch thick. Cut rounds using a 2-inch fluted cookie cutter and place 1-inch apart on the prepared baking sheet. Repeat with scraps and remaining dough.
Bake for 8 to 12 minutes (depending on thickness). The cookies should be just set and you should start smelling the aroma of baking chocolate. Remove to a cooling rack. Once all cookies are cool, prepare the filling.
To prepare the filling:
In a small mixing bowl, whisk together the powdered sugar, cocoa and salt. In a medium mixing bowl, beat the butter and corn syrup until smooth. In a small measuring cup combine the vanilla and espresso powder until dissolved. Add the vanilla mixture to the creamed butter and blend until incorporated. Add half the cocoa mixture and beat on low until incorporated. Add the remaining cocoa mixture and beat until blended, scraping down the sides and bottom of the bowl as needed.
Scoop the filling into a zipper bag, or pastry bag. Cut the corner to create a ¼-inch opening.
Turn half the cookies over with the bottom facing up. Pipe the filling onto the cookies to cover. Top with a second cookie, bottom side down against the filling. Press gently to adhere.
Refrigerate to set the filling, if desired. Store cookies in a cool location in an airtight container for up to 5 days. Refrigerate for longer storage.
Freeze cookies before filling, if desired. Thaw for at least 4 hours before filling.