I'll be perfectly honest — I'm not much of a baker. The precise measurements, the exact timing of everything, it's not really my game. But, for this cake, I make an exception. For chocolate (especially dark chocolate) you should always make an exception. Rich, dark, fudgy, with a little crunch from the walnuts, let's just say this is not a chocolate cake for the I-just-kind-of-like-chocolate person.
Preheat the oven to 350 degrees.
Place the chocolate and butter in a large heatproof bowl over a pot of steaming water. Stir to until the mixture is smooth and melted. Set aside.
In another large heatproof bowl, combine the eggs, vanilla extract and sugar. Place the bowl of the pot of steaming water, making sure that the water and the bottom of the bowl do not touch.
Use an electric beater to beat the egg mixture until it becomes very pale and thick (you should be able to make a ribbon-like trail when lifting the beater).
Remove the bowl from the heat and let it cool for a few minutes. Next, gradually fold in the chocolate a couple spoonfuls at a time — take your time here, you don't want to deflate the mixture. Once all of the chocolate has been integrated, mix in the walnuts.
Pour the final mixture into the prepared springform pan and bake in the oven for about 35 minutes (you want the top to have formed a light crust but for the interior to still be moist).
When ready to serve, remove from the pan and dust with powdered sugar.