Preheat the oven to 350F.
In a medium bowl, mix the softened butter and sugar until combined. Mix in the egg yolk and vanilla until well combined.
Sift the flour, baking soda, cocoa powder, espresso powder and salt. Fold in the chocolate chips and chopped chocolate.
Chill for 20-30 minutes while the oven preheats.
Line a baking sheet with parchment paper.
Scoop the dough into two portions (or 3 for smaller size) and place on a baking sheet at least 2-inches apart. Press a few extra chocolate chips on the top.
Bake for 13 minutes - when the edges are firm and set and the center is just set.
Allow the cookies to cool on the baking sheet for about 10, the remaining heat will continue baking the cookie.