Coarsely chop the chocolate and place over a hot water bath.
Melt the chocolate, stirring constantly. Set aside to cool. Beat the butter and brown sugar until creamy. Stir in the eggs and melted chocolate. Mix the flour, baking powder, 1 pinch of salt and vanilla. Preheat the oven to 200°C (approximately 400°F).
Chop the walnuts. Fold the walnuts and flour mixture into the batter. Grease the baking pan with butter or oil. Pour the batter onto the pan and smooth. Bake for about 30 minutes, until a toothpick inserted into the center comes out clean. Remove from the oven and cool on a wire rack. Cut into 5 cm rectangles.
Cut out a template (such as a star) from parchment paper or cardboard. Place the template on each brownie and sprinkle with powdered sugar. Place each brownie in a baking cup. Serve immediately.