Set a heatproof bowl over a saucepan of simmering water, ensuring that the bottom of the bowl does not touch the water's surface. Add the butter pieces, bittersweet and unsweetened chocolate. Heat, stirring occasionally, just the chocolate is almost melted, then remove from heat and stir until it melts completely. Remove the bowl from the heat, and set it on the counter to cool to room temperature.
In a small bowl, sift together the flour, cocoa, salt and baking powder. Set aside.
Place a rack in the center of your own, and preheat the oven to 350 degrees F. Line two baking sheets with parchment paper or silicone mats.