Preheat oven to 325 degrees F. Line an 11 by 16-inch baking sheet with parchment paper or use a silicone baking mat.
Melt 4 ounces of the chocolate chunks with the butter. Set aside the mixture to cool slightly.
In a medium bowl, sift together the flour, baking soda and cocoa. Whisk in the espresso powder and salt.
In the bowl of an electric mixer, combine the chocolate mixture, sugars, egg and vanilla at medium speed. Reduce the speed to low and add the flour in thirds. Turn off the mixer and fold in the remaining 4 ounces chocolate chunks.
Using a spoon or an ice cream scoop, form the dough into 1-inch balls and place on the prepared baking sheet, spacing 2 inches apart. Bake for 10 to 12 minutes or until the cookies have spread into disks and the tops are crackled. They will still be soft. Set the baking sheet on a cooling rack and let the cookies cool for about 2 minutes. Remove with a spatula to a cooling rack to let cool completely.