Sift together the flour, salt, and baking soda. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter on medium-low speed until smooth, about 2 min. Add the sugar in a steady stream and mix for another 2 min. Add the egg and vanilla and mix until well combined, scraping the bowl as needed. Reduce the speed to low and add the dry ingredients in two additions, mixing just until combined. Remove 2 cups less 2 Tbs. of the dough and set aside.
Dissolve the espresso powder in the boiling water and set aside briefly to cool. Then mix the espresso and cocoa powder into the remaining dough. Reduce the mixer speed to low, add the warm melted chocolate and mix just until thoroughly combined.
