1. Place the chocolate couverture and butter in a large bowl, then set it over a saucepan of simmering water (bain-marie). Stir with a silicone spatula until melted, then remove from the heat.
2. In a mixer, beat the eggs with both types of sugar until light and fluffy. Add the vanilla.
3. Gently fold the melted butter and chocolate mixture into the egg mixture. Add the sifted flour and cocoa powder, incorporating them well with the help of a spatula.
4. Add the finely chopped walnuts and stir them into the mixture very briefly. Line the base of a 20cm square baking pan with non-stick parchment paper.
5. Pour the brownie batter into the pan and bake at 175 degrees Celsius for 20-25 minutes.
For the Ganache:
1. Place the chocolate couverture and heavy cream in a bowl, then set it over a saucepan of simmering water (bain-marie).
2. Once melted, remove from the heat and add the butter. Cover with plastic wrap. Let it cool, then use a piping bag to decorate the brownie with the chocolate ganache.