Pre-heat your oven to 375 °F. Grease (with butter) a 10 inch spring form pan, cutting out a parchment circle for the bottom as well as a liner.
Pour the batter into the prepared pan. Cook in the oven for 22-25 minutes, until the middle is sightly jiggly when moved and the top of the cake has a slight crust. The less you cook it, the fudgier it will be! Make sure that the temperature in the middle reaches 165°F for raw egg safety.
Melt together the chocolate and the butter until smooth. Pour over the cake and let harden slightly.
This cake makes a lot of servings as it is really rich!