Preheat oven to 180ºC . Grease a 23 cm springform pan. Line bottom with a round of parchment or wax paper. In a 2 litre glass bowl, combine 0.75 cup of sugar and one third cup water. Heat in microwave on HIGH 1.5 minutes, or until boiling. Stir in semisweet and unsweetened chocolate and butter until melted. In a medium bowl, beat eggs and remaining quarter cup sugar with an electric mixer on high speed until double the volume, about 3-4 minutes. Stir in this mixture into the chocolate mixture until well blended. Turn into prepared pan. Set springform pan in a larger rectangular baking ppan, and water up to an inch on the springform pan. Bake for 45 minutes, or until cake is set. Remove pan from water, let cool in pan and refrigerate.