Melt the butter, milk, sugar, cocoa and vanilla extract into a large saucepan. Stir and brig to boiling point. Let it simmer for 15 seconds and turn off the heat.Break up the chocolate for the glaze into a small metallic bawl and pour over 8 tbsp of the hot cocoa mixture. Set aside.Immerse the saucepan into a few inches of cold water to speed up the cooling process. Stir in from time to time.
Prepare and line the baking pan. Preheat oven to 350F.Separate the eggs. Beat the egg whites with a pinch of salt until stiff peaks with a hand or standing mixer. By now the cocoa batter should be cooled. Beat in the egg yolks one by one.Add the flour sifted with the baking powder and lastly the beaten egg whites. Incorporate everything with delicate moves.
Transfer the batter into the baking pan and cook for about 25 minutes at 350F. Take it out of the oven and place instead the broken chocolate bowl to which you add 6 more tbsp of milk. Let it melt for a few minutes.Spread the glaze over the cake and sprinkle with chopped walnuts all over the surface or just for 1/2 of it. Let it cool completely and cut it to your linking.
Notes
Keep it covered with cling film for about 3-4 days in the fridge or on the kitchen counter.