Step 2: Place 12 ounces chopped bittersweet chocolate and 3/4 cups unsalted butter in a large heatproof bowl over a pot of steaming water. Stir to until the mixture is smooth and melted. Set aside.
Step 3: In another large heatproof bowl, combine 5 eggs, 1 teaspoon vanilla extract and 1 1/2 cups sugar. Place the bowl of the pot of steaming water, making sure that the water and the bottom of the bowl do not touch.
Step 4: 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.)
Step 5: 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 3 1/2 ounces of walnuts.
Step 6: Pour the final mixture into a 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.
Step 7: When ready to serve, remove from the pan and dust with powdered sugar.