Preheat the oven to 350 degrees and line rimmed cookie sheets with parchment paper.
In a medium bowl, whisk together the flour, salt, espresso, cocoa, ancho, and baking powder until well combined. Set aside.
Melt the chocolate in a medium heatproof bowl set over a pot of simmering water, stirring occasionally until all the chocolate has melted. Remove the bowl from the heat and let cool slightly.
Meanwhile, in the bowl of an electric mixer, cream together the butter, brown sugar and 1/2 cup white sugar until light and fluffy. Add the eggs and mezcal (if using) and beat on medium speed until just combined. Add the flour mixture and beat on low until the flour has been worked into the batter enough that it's no longer resting on the surface, don't overmix. Add the melted chocolate and continue to beat on low until the chocolate is completely incorporated.
Cover the bowl and refrigerate for 20 to 30 minutes until the mixture has thicken and will hold its shape once rolled. Use a cookie scoop to portion out the batter. Roll each cookie into a ball, then roll them in the reserved 1/2 cup sugar to coat. Place the cookies on the prepared cookie sheets leaving about 1 1/2 -inches between the cookies. Using the bottom of a glass, slightly flatten the cookies. Sprinkle a small pinch of the cocoa nibs over the top of each of the cookies. Bake for 12 to 14 minutes, just until the edges of the cookies are set, rotating the sheets halfway through the baking time. The middle of the cookies will still be very soft, they firm up as they cool. Place the cookie sheets on wire racks to cool for 5 minutes. After 5 minutes, lift the whole sheet of parchment with the cookies off the cookie sheets and let cool completely before removing them from the parchment. Store them in an airtight container. They also freeze very well.
Cocoa nibs are sold in specialty stores or on the internet at The Spice House.
Cookie scoops are available in most housewares sections or cooking stores and on the internet.