Preheat oven to 325 degrees Fahrenheit.

Combine flour, baking powder, and salt in a sifter.

Sift the dry ingredients onto wax paper and set aside.

Heat 1 inch of water in a double boiler over medium heat.

Place 6 ounces of semisweet chocolate, unsweetened chocolate, and butter in the top half of the double boiler.

Stir the chocolate and butter until completely melted and smooth (about 6 minutes).

Transfer the melted chocolate mixture to a 1-quart bowl and set aside.

In a separate bowl, combine eggs, sugar, and vanilla extract.

Beat on medium speed with an electric mixer for 4 minutes until soft.

Scrape down the sides of the bowl and continue to beat on medium for 1 minute until incorporated.

Scrape down the bowl again.

Add the sifted dry ingredients and the remaining 4 ounces of semisweet chocolate.

Mix on low speed until just incorporated (about 30 seconds).

Remove the mixer and use a spatula to finish mixing the dough until thoroughly combined.

Portion out 12 cookies onto 3 nonstick baking sheets, using a heaping tablespoon of dough for each cookie.

Space the cookies evenly on the baking sheets.

Place the baking sheets on the top and center racks of the preheated oven.

Bake for 9-10 minutes, rotating the sheets from top to center halfway through the baking time.

Turn each sheet 180 degrees at the halfway point.

Remove the cookies from the oven and allow to cool to room temperature on the baking sheets (about 30 minutes).

Store the cooled cookies in a tightly sealed plastic container.