Preheat oven to 350º.
Line inside of 9-inch springform pan with nonstick foil and wrap a piece of foil around outside, too.
Heat butter and chocolate in the microwave until melted. Whisk till smooth.
Whisk in the eggs and sugar.Add vanilla and pour into prepared pan.
Place the pan in a larger roasting pan and fill the larger pan with hot water to come about halfway up the sides of the pan.
Bake 40-45 minutes and remove from oven, allowing cake to cool in the water bath.
Once cool, place cake pan in freezer.