In a small bowl, whisk together flour, cocoa powder, baking powder, and salt.
In a medium-sized mixing bowl, beat butter and sugar until light and fluffy. Mix in egg whites, egg, and vanilla extract until thoroughly combined.
Melt 3 ounces of the dark chocolate in the microwave or in a double boiler. Mix melted chocolate into the wet ingredients a little bit at a time until fully combined.
Mix in dry ingredients until just combined.
Fold 4 ounces of the remaining unmelted chocolate into the batter.
Scoop tablespoon-sized balls of dough, three inches apart from one another, onto cookie sheets lined with parchment paper or silicone mats. Press dough balls down slightly with your thumb to flatten the tops.
Press the remaining 2 ounces of chocolate pieces into the tops of the cookies so that there will be some chocolate visible at the top of each cookie after the bake.
Bake for 10 minutes or until the edges of the cookies are set but the middles are still gooey. Remove from the oven and let sit on the cookie sheets for two minutes before transferring to a cooling rack.
Serve warm or, once cooled, store in a loosely-covered container for up to four days.
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Notes
Make sure to use tablespoon-sized balls of dough. If the cookies are too small and thin, they will end up being crispy instead of fudgy.
Be careful not to over-bake the cookies. If you leave the cookies in the oven too long, they will be crispy instead of fudgy.