Preheat oven to 375°F.
In small bowl, whisk together flour, cocoa powder, baking powder and salt.
In medium-sized mixing bowl, beat butter and sugar until light and fluffy. Mix in egg whites, egg and vanilla until thoroughly combined.
Melt three ounces of dark chocolate in microwave or in double boiler. Mix melted chocolate into wet ingredients a little bit at a time until fully combined.
Mix in dry ingredients until just combined.
Fold 4 ounces of remaining un-melted chocolate into batter.
Scoop tablespoon-sized balls of dough onto cookie sheets lined with parchment paper or silicone mats, spacing cookies three inches apart from one another. Press dough balls down slightly with your thumb to flatten tops.
Press remaining 2 ounces of chocolate pieces into tops of cookies so that there will be some chocolate visible on top of each cookie after they bake.
Bake 9 minutes or until firmly set. Remove from oven and let cool on cookie sheet for two minutes before transferring to cooling rack. Serve warm or, once cooled, store in loosely-covered container for up to 4 days.