Chop the chocolate into pieces and melt with the butter in a bain marie. In a mixing bowl whip 175 grams of sugar along with the egg yolks until light and fluffy. Fold the egg mass into the chocolate. Whip the egg whites with the rest of the sugar. Combine all the ingredients.
Spread the mixture in a greased 25 x 26 x 6 cm ceramic oven dish lined with baking paper. This will give a depth of 3 to 5 cm. Bake for 25 minutes at 180° C. While the lower part of the cake is baking make the same mixture one more time (alternatively make in one go and divide into two). When the baked cake is cold spread the unbaked second mixture on top. Put the cake in the freezer for 3 hours or until totally frozen. Remove from the freezer 30 minutes before serving. Cut into 4 x 6 cm pieces using a large warm knife.
The cake can easily be made in advance. Serve with fresh fruit such as strawberries, or drizzle with cocoa powder and decorate with a chocolate ribbon.
To make the chocolate ribbon, first make a template by cutting a 2 x 10 cm hole in a piece of card. Place the template on a sheet of baking paper. Pour some chocolate carefully into the template and scrape off any excess with a spatula. Before the chocolate sets completely, drape the baking paper and chocolate over a rolling pin or a kitchen paper roll to give it a curl.