Preheat the oven to 350 degrees F. Rub the pan generously with butter to grease.

Separate the eggs first. Put the whites in the bowl of a stand mixer with a whisk attached (or a large mixing bowl) and put the egg yolks in another bowl. Let them sit while you prepare the chocolate.

Combine the chopped chocolate and 7 tablespoons of butter in a microwave safe bowl and heat on high, stirring after 60 seconds, then every 30 seconds or until melted and smooth. You can do this in a saucepan if you prefer.

Meanwhile, in the bowl of a stand mixer with the whisk attached, beat the egg whites until stiff. Scrape them onto a plate or into a bowl to wait.

In the same bowl you used for the egg whites, beat the granulated sugar and yolks until pale and yellow. Scrape in the melted chocolate and beat until mixed, then stir in the cornstarch and salt (if using) and vanilla (if using). Lastly, fold the beaten egg whites.

Transfer the batter to the springform pan and bake for about 20 minutes. Do not overbake.

Allow the cake to cool for 30 minutes. It will collapse as it cools. Loosen from edges, remove springform and serve with your favorite berries.