Melt chocolate with butter over simmering water. Cool and set aside.Beat egg whites until stiff peaks form.
Beat egg yolks in another bowl with 1 cup sugar until thickened and light lemon colored.Stir in chocolate mixture to yolks. Add half of whites and stir. Fold in remaining whites.
Pour batter into an 8 inch (17 cm) springform pan, previously buttered. Set pan on a baking sheet.
Bake in preheated oven to 325°F (160°C) for 50 minutes approximately. Remove from oven and cool for 10 minutes. Run a knife between the sides of the pan and the cake. Remove the sides of the pan and let the cake cool completely.
Process or blend strawberries or raspberries with remaining granulated sugar and force through a strainer.
Before serving, run a knife under the cake and transfer it to a cake plate. Sift confectioner’s sugar over the cake.