In a microwave safe bowl combine the butter, 6 ounces of chopped chocolate & espresso powder. Heat in the microwave for 20 to 30 second intervals, stirring the chocolate in between each heating. Repeat until chocolate is completely melted & smooth. Set aside.
In a large mixing bowl combine the light brown sugar, eggs & vanilla and beat on medium speed (using a hand held mixer or stand mixer) until smooth. Scrape down the sides of the bowl as needed. Add in the melted chocolate mixture and continue beating just until combined.
In a separate bowl combine the dry ingredients; flour, cocoa powder, baking powder & salt. Give the dry ingredients a quick whisk just to combine them. Slowly add the dry ingredients into the chocolate batter, mixing on low speed until the cookie dough forms. Cover the bowl with plastic wrap and chill in the refrigerator for an hour.
Preheat the oven to 325 degrees and line a baking sheet with parchment paper. Now you can cut 4 ounces of the baker's chocolate into smaller squares, one to cover each cookie and cut the marshmallows in half. Once the dough has had a chance to chill, use a spoon or cookie dough scoop to scoop out balls of dough and place them on the baking sheet giving them two inches of space in between each cookie. Bake for 10 minutes. Remove cookies from the oven and carefully place a single square of chocolate on the surface of each cookie, pressing down slightly. Place a marshmallow on top of the chocolate square. Return cookies to the oven to bake for an additional 3-5 minutes. Remove cookies from oven. At this point you can grate the remaining 2 ounces of chocolate over the cookies or you can use a kitchen torch to toast the marshmallows. Allow cookies to cool for 5 to 7 minutes before transferring them to a wire rack to cool completely.