The following recipée should make somewhere around 20 pieces of cake.

340 g of your favourite 70% chocolate (I use Amedei's Chuao)
275 g of unsalted butter
210 g of sugar
100 ml water
5 whole eggs

Grease a square ovenproof 250 cm2 pan and line it with paper. Place a cloth towel in a roasting pan and fill it halfway up with water. Heat the oven to 150 degrees Celcius. Melt the butter and chocolate over a bain-marie. Heat 70 g of sugar in the water untill it's dissolved. Whip the eggs with 140 g of sugar untill it has quadrupled in size. Now mix the sugar water into the chocolate blend and quickly but carefuly fold into the eggs. Pour the batter into the pan and place it in the water filled tray. Bake at 150 degrees Celcius for about 50 minutes. Let the cake cool before tiping it out of the pan. Cut it into small squares and top it off with a fine coat of cocoa powder.

Enjoy
istarios