1. Melt the chocolate and half of the butter over a hot water bath. Remove from the heat and let cool slightly.
2. Separate two eggs. Beat the egg whites with 50 grams (approximately 1.75 ounces) until sugar until stiff.
3. Beat the remaining butter with the remaining sugar until creamy.
4. Mix in the remaining eggs and egg yolks. Stir until the mixture is light and creamy.
5. Stir in the chocolate. Fold in the walnuts.
6. Add the flour and mix well.
7. Carefully fold the egg whites into the batter.
8. Line a baking sheet with parchment paper. Form an edge around the pan with aluminum foil. Pour the batter onto the pan and smooth.
9. Bake for about 25 minutes at 180°C (approximately 350°F). Remove and let cool.
10. Cut into pieces and serve.