The recipe, adapted from my Tuscanian Cookbook by Stephanie Alexander and Maggie Beer:

200g bittersweet chocolate, chopped
200g butter
100g flour
200 g Sugar
5 – 6 eggs, depending on the size

Preheat the oven to 180° and butter a 26cm baking form  (or two smaller baking forms like in the picture). Melt the butter and the chocolate in a bain-marie (double boiler) stirring often until smooth. Set it aside to cool for 5 minutes. In the meantime whisk the eggs and the sugar until creamy. Add the flour and stir to mix well. Finally add the chocolate butter.
Pour the batter into the prepared baking form and bake for about 25-30 minutes. The top should be slightly crackled and the center still creamy. Let the cake cool completely before cutting. Enjoy!