Melt the chocolate with half the butter over a pan of hot water and set aside to cool slightly.
Separate two eggs. Mix the egg whites with 50g sugar (approximately 2 ounces) until cut resistant.
Beat the remaining butter with the remaining sugar until creamy.
Add the remaining eggs and yolks to the butter-sugar and mix well until the mixture is light and creamy. Add the chocolate, followed by the walnuts, and stir well. Finally, mix in the flour and cocoa powder.
Carefully fold the egg whites into the cake batter.
Line a baking sheet halfway with parchment paper. Slide 3-4 layers of wide strips of aluminum foil halfway under the parchment paper. Fold up so that it forms an edge.
Pour in the cake batter evenly and bake in a preheated oven at 180°C (approximately 350ºF) for about 25 minutes. Allow to cool on the baking sheet.
Cut and serve in about 20 pieces.