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 banana filling.