Melt the semi sweet and unsweetened chocolates and butter in a large heatproof bowl (I used stainless steel), placed over a saucepan of simmering water. Remove from heat and whisk in the cocoa powder and sugar. Next, whisk in the vanilla extract. Then, whisk in the eggs, one at a time, whisking well after each addition. Finally, stir in the flour, salt, and walnuts.

Pour into the prepared pan, smoothing the top with the back of a spoon or offset spatula. Bake for about 23 -25 minutes or until a toothpick inserted into the center comes out with moist crumbs clinging to it. Remove from oven and let cool on a wire rack.

Melt the white chocolate over a pan of gently simmering water. Cut the brownies into portions and drizzle each portion with the white chocolate. Leave to set for a few minutes before serving, dust the brownies with the remaining cocoa powder.
Serve at room temperature or chilled. These Brownies freeze very well.