In a large bowl, melt butter, bittersweet chocolate and 1 cup of chocolate chips. I do this in the microwave, beginning with 30 second increments and stirring until totally melted. Set aside and let cool completely.
In a small bowl, combine flour, cocoa powder, salt and baking powder. Set aside.
In the bowl of your electric mixer, add sugar and eggs, beating on high until light and fluffy, about 2 minutes. Add in vanilla extract and melted chocolate/butter, beating for another 1-2 minutes until combined. Scrape down the sides of the bowl and the whisk or beater attachment, then remove it and stir in the dry ingredients with a large spatula until thoroughly dispersed by just combined. Fold in remaining chocolate chips. Cover the cover with plastic wrap and place in the fridge for 4 hours (or even overnight!).
Preheat oven to 350 degrees F. Place parchment paper (or a silpat!) on an ungreased baking sheet. Wet your hands with water then begin to roll the dough into 1-inch balls, placing them about 2 inches apart. Do this quickly and if you are doing the cookies in batches, refrigerate the dough in between rounds. Sprinkle a bit of salt on top of each dough ball, then bake for 9-10 minutes. Remove, sprinkle a tiny bit of salt again, and let cool for at least 10 minutes on the baking sheet. The should be completely cool before removing them! Once removed, place on wax or parchment paper to cool completely. Once totally cooled, if desired you can dip half of each cookie in melted chocolate then cover in sprinkles. These store pretty well but are fairly fragile.