Butter 10-inch springform pan.
Line bottom of the pan with a parchment round; butter parchment.
Wrap 3 layers of heavy-duty foil around the outside of the pan, bringing it up to the top of the rim.
Combine 1 cup of water and sugar in a small saucepan and bring to boil over medium heat. Stir till sugar dissolves, then simmer 5 minutes. Remove from heat.
Melt 9 tablespoons of butter and 18 ounces of chocolate in the microwave.
Whisk smooth.
Whisk sugar syrup into chocolate, and cool slightly.
Add eggs to chocolate mixture and whisk till well blended.
Pour batter into prepared pan.
Place cake pan in a large roasting pan and add enough hot water to roasting pan to come halfway up sides of cake pan.
Bake cake until the center is just set about 50 minutes.
Remove from water bath; transfer to rack. Cool completely in pan.
