Prepare a double broiler. Put about 1" of water in a small saucepan and place at medium-low heat. In a glass bowl that will fit at the top, place the two sticks of salted butter, 18oz. of chocolate, and 1/4 cup of water. Stir until the chocolate has completely melted. Remove the bowl from heat. Set a timer to allow this to cool for 10 minutes.
Preheat your oven to 350 degrees.
In a standing mixer, with a whisk attachment, place the 6 eggs, 1/2 cup sugar, 1/4 cup unsweetened cocoa powder, salt and pure vanilla extract in the bowl. Whisk at medium speed for 4 minutes. When the 4 minutes are up, your chocolate mixture should be cooled. Slowly whisk in the chocolate mixture for one additional minute. Pour the mixture into your prepared pan or cupcake liners, and bake for between 15-35 minutes, depending on if you're making cupcakes, and the depth of the batter in the pan you're using.
When you first take the cake out of the oven, it will be slightly cracked at the outer edge, and will rise above the pan.When it cools, the cake will sink. This is exactly what should happen. Once it's cooled, you can remove the cake from the pan and cover it in plastic wrap to refrigerate for one day, unless you are serving it the same day.If you'd like to decorate the flourless chocolate cake with fresh berries, so that it looks more like a torte, I recommend drizzling the top of the cake with fresh ganache.