These cookies are chocolate on chocolate and so good they will spoil you for every other cookie out there.
Melt the unsweetened and bittersweet chocolates together in a microwaveable bowl: Cook 1 minute, then stir, then continue to cook in 30 second increments, stirring after each, until just melted. Set aside to cool completely. Will cool faster if removed to a metal bowl and set over a rack so air can circulate. Stir now and then.
Place the butter and sugar in the bowl of a stand mixer and mix until combined. Add the eggs 1 at a time, mixing well after each. In another bowl, mix the flour, baking powder, salt, and espresso powder. Add to the egg mixture and mix, stopping to scrape down, until just incorporated.
Add the melted chocolate, mixing well on low, stopping to scrape down. Stir in the walnuts and chopped chocolate pieces.
Push, shove and pour the mixture onto a parchment-lined baking sheet or tray, even into a rectangle as best you can. Refrigerator for 5 to10 minutes until slightly, but not completely, hardened.
Using a knife, divide the mixture into 4ths, then each quarter into nine rough squares (three squares by three squares.) With your hands, roll each of the squares into a ball and evenly space them onto a parchment-lined baking sheet. Make sure to leave room to spread, about three inches between cookies.
Bake in a 350 degree F. oven for about 10 to 12 minutes, rotating front to back and top to bottom halfway through baking time. Look for the cookies to be firm on the edge, but still look slightly underbaked in the cracks. Remove parchment containing the cookies from the hot pan, immediately and leave to cool.
Cookies may be frozen in raw state; thaw and bring to room temperature before baking.
Note: Instructions have been slightly adapted.