Combine the crumbled dark chocolate and butter in a container in a water bath on the stove and stir until the mixture melts and homogenizes.
Pour in the sifter flour with the baking powder and ground almonds. Lay out baking paper in a medium-sized rectangular baking dish and pour in the mixture. If desired, distribute pieces of crumbled white chocolate and push them in carefully to sink in the mix.
Bake the brownie with chestnuts for about 25-30 min. in a preheated 356°F (180 °C) oven. Let it cool in the tray before cutting it into pieces.