Coarsely chop the chocolate and melt over a hot water bath, stirring frequently. Set aside to cool briefly. Beat the butter and brown sugar until creamy. Stir in the eggs and melted chocolate. Preheat the oven to 200°C (approximately 400°F).
Mix the flour, baking powder, 1 pinch of salt and the vanilla. Chop the walnuts. Fold the flour mixture and walnuts into the butter mixture. 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 out small rectangles. Use parchment paper to cut out a template (such as a star). Place the template on each brownie and sprinkle with powdered sugar. Place each brownie in a baking cup. Serve immediately.