Preheat the oven to 325 degrees F. Combine the flour, baking powder and salt.
Heat 1 inch of water in the bottom half of a double boiler over medium heat. With the heat on, place 6 ounces semisweet chocolate, the unsweetened chocolate, and butter in the top half of the double boiler. Use a rubber spatula to stir the chocolate and butter until completely melted and smooth, about 6 minutes. Set aside until needed.
Place the eggs, sugar and vanilla in the bowl of an electric mixer. Beat on medium 6 minutes until soft, scraping down the sides of the bowl as you go. Add the melted chocolate and beat on medium for 1 minute until incorporated. Add the sifted dry ingredients and the remaining 4 ounces of semisweet chocolate and mix on low speed until just incorporated, about 30 seconds. Remove the bowl from the mixer and use a rubber spatula to finish mixing the dough until thouroughly combined.
Using a heaping tablespoon of dough for each cookie, portion 12 cookies onto each baking sheet. Bake for 9 to 10 minutes.