Preheat oven to 350 degrees. Butter 9-inch round cake pan. Line bottom with round of parchment paper and butter the paper.
In food processor combine chopped chocolate. Process until finely chopped.
In saucepan bring 1/2 cup water and sugar to boiling. Stir to dissolve sugar.
With food processor running, add boiling sugar syrup to chocolate through feed tube. Add butter, 2 tablespoons at a time. Add eggs. Process until smooth.
Pour the chocolate mixture into prepared pan. Place pan in roasting pan on rack in center of oven. Pour enough hot water into roasting pan to reach halfway up sides of pan. Bake 25 to 30 minutes until knife inserted in center comes out with just a few specks of chocolate and cake begins to pull away from sides of pan. Transfer cake pan to wire rack. Cool 10 minutes.
Using a small, sharp knife loosen cake from pan sides. Cove cake surface with plastic wrap, and invert onto baking sheet. Lift off pan, remove parchment. Invert a cake plate over cake, turn plate and baking sheet so that cake is right side up. Remove plastic wrap and cool.