How to Make It
Preheat the oven to 350°. Wrap the outside of an 8-by-3-inch round springform pan in heavy-duty foil, then generously butter the inside of the pan. Set the springform in a small roasting pan.
In a saucepan, combine the butter with the granulated sugar and water and bring to a boil over moderate heat, stirring. Remove from the heat. Add the chocolate and stir until smooth; let cool.
In a medium bowl, whisk the flour with the eggs until blended. Add to the chocolate batter and whisk until smooth. Pour the batter into the prepared springform pan. Pour enough hot water into the roasting pan to reach halfway up the side of the springform. Bake the cake in the oven for about 1 hour and 15 minutes, or until the top is crusty and a cake tester inserted in the center of the cake comes out with a few moist crumbs attached. Let the cake cool in the springform on a rack for 20 minutes. Remove the foil and the side of the pan and let the cake cool completely. Dust the cake with confectioners' sugar just before serving.