Melt the bittersweet chocolate, stir in the espresso powder and set aside.
Whisk together the flour, baking powder and salt in a small bowl, set aside.
Place the butter and sugar in the bowl of a stand mixer and mix until well combined.
Add the eggs 1 at a time, scrape down the bowl as necessary.
Stir in the flour mixture until just combined.
Add the melted chocolate mixture and mix until just combined.
Fold in the toasted walnuts and the chocolate pieces.
Let the dough rest about 30 minutes or until it is firm enough to scoop easily.
Preheat oven to 325 degrees F.
Scoop small mounds (2 tbsp cookie scoop) of the dough onto a parchment paper lined baking sheet.
Sprinkle the top of each cookie with coarse sea salt and bake for about 10 minutes. Cookies should be set at the edges and still soft in the center.
Let cookies cool on the pan for several minutes before moving to a wire rack to cool completely.
