In the bowl of the stand mixer cream the butter, then add the brown sugar and beat the mixture until it is light and fluffy.
Beat in the yolk, then add the vanilla and the espresso mixture a little at a time, beating between additions. Continue to beat the mixture until it is combined well.
Add the salt and the flour, and beat the mixture until is combined well.
Spread the batter evenly in a jelly-roll pan, 15 1/2 by 10 1/2 by 1 inch, and bake it in the middle of a preheated 350° F. oven for 15 to 20 minutes, or until it pulls away slightly from the edge of the pan.
Spread the melted chocolate evenly over the baked layer and sprinkle the chopped cashews over it. Let the mixture cool in the pan on a rack, cut it into 48 bars, and chill it for 15 to 20 minutes or until the chocolate is firm.