Preheat the oven to 350 degrees F.
Spray a 9-inch springform pan with cooking spray. Line the bottom of the pan with a circle of parchment paper.
Set up a double boiler on the stove over low heat and heat until the water is barely simmering. Place the chocolate and butter in the double boiler and melt, stirring occasionally. Once the chocolate and butter are melted, remove from the heat and stir in the water, vanilla and 1 1/2 cups of the granulated sugar. Whisk in the egg yolks.
In a separate bowl beat the egg whites with an electric mixer until frothy. Add remaining 1/2 cup of the granulated sugar and beat until soft peaks form. With a rubber spatula, gently fold the egg whites into the chocolate mixture. Bake for 45 minutes. Serve.