In a medium bowl, combine flour and cocoa powder. Set aside.
In a stand mixer fitted with a paddle attachment, cream together the butter sugars, salt, baking powder, baking soda, espresso powder and vanilla extract. Mix over medium/high speed until just combined, no longer than seconds. This will ensure a crunchy wafer.
Reduce the speed to medium and add egg yolks, one at a time, beating well after each addition. Stop the mixer and use a rubber spatula to scrape the bowl. Turn the mixer on to low speed, and add the flour and cocoa powder mixture. The mixture will be stiff, but continue mixing on low speed until combined.
Divide the dough in half, flatten each portion into a rough square, then wrap with plastic wrap and place in the refrigerator for about 30 minutes to an hour.
Preheat the oven to 350° Fahrenheit, and line two cookie sheets with parchment paper. Set aside while you roll out the cookie dough.
Tape a sheet of parchment paper on your counter and dust with a light coating of cocoa powder. Place the dough on top of the dusted parchment, dust with a light coating of cocoa powder, and then place another piece of parchment over the dough. Roll the dough to about 1/8” thickness. Please be sure to measure out the dough, unless you like thick cookies.
Using a 2” round cutter, cut circles in the dough, making sure to cut as close as possible. Place the cut circles onto the prepared cookie sheet. These cookies don’t really spread while baking, so feel free to place them close together. Not touching, but close. Gather up the dough scraps, pat them to a rough square, and re-roll, arranging them onto the cookie sheet.
Place the dough in the oven and bake the cookies for about 12 minutes, rotating the pan halfway through. Remove the cookie sheet from the oven and allow the cookies to cool. No need to transfer them onto a wire rack. Cool the cookies completely before adding the filling.
While the cookies are cooling, make the Dulce de Leche filling.
Make the filling:
In a bowl of a stand mixer fitted with a paddle attachment over low speed, cream together the shortening/butter, powdered sugar, and salt until combined. Scrape down the bowl as needed.
Add the vanilla extract and dulce de leche and increase the speed to medium/high. Mix the frosting using this speed for about 10 minutes or so. If you prefer a stiffer frosting, add more confectioners’ sugar. You can also add more dulce de leche, 'cause one can never add too much.
Transfer the filling into a pastry bag fitted with a large plain tip. Alternately, you can portion the filling with an ice cream scoop or a teaspoon. Flip half of the cookies so the very flat portion is upright. Onto each of these cookies, spoon or pipe as much filling as you'd like. Top with another cookie and press down on the cookie until the filling reaches the edges.
At this point you can transfer the cookies to an airtight container and refrigerate to help the filling adhere. Or you can leave at room temperature on a cookie sheet as the filling adheres on its own, which is how I roll.
Cookie dough adapted from Bravetart
Makes over 60 sandwich cookies