12 ounces bittersweet chocolate, roughly chopped
1/2 cup water
2/3 cup sugar, divided
2 sticks unsalted butter, at room temperature, cut into small pieces
5 large eggs
Preheat oven to 350°F.
Spray a 9-inch round cake pan with nonstick spray with flour; set aside.
Combine water and 1/3 cup sugar in a heavy saucepan over high heat, stirring until sugar melts.
Bring to a boil, and cook 2 minutes.
Remove saucepan from heat; slowly add chocolate, stirring until completely melted.
Stir in butter, a few pieces at a time, until completely melted.
Combine eggs and remaining 1/3 cup sugar in a medium bowl; beat with an electric mixer until pale yellow and very frothy.
Slowly stream chocolate-butter mixture into egg mixture, stirring until smooth and satiny.
Pour batter into prepared pan.
Place cake pan inside of a larger roasting pan; pour hot water around cake pan until water reaches halfway up sides of cake pan.
Carefully transfer pans to oven.
Bake 28 to 30 minutes until just firm.
Remove cake pan from roasting pan and let cool 30 minutes.
Carefully invert cake onto a large plate before serving, or serve directly from pan