Preheat oven to 180 degrees C (350 degrees F).
Line a 18 -20 cm (8 inch) cake pan with a circle of parchment paper and then grease well the pan inside.
In a small bowl combine starch, baking powder and salt.
In a heatproof bowl combine butter and two kinds of chocolate. Melt them together over a pan of gently simmering water.
In a large mixing bowl with an electric mixer beat eggs and sugar for 5-6 minutes until the mixture becomes pale and increases significantly its volume. Add vanilla and beat shortly to incorporate it. Using a rubber spatula gently fold the chocolate mixture into the eggs. At the end gradually add the dry ingredients mixing carefully with the spatula, until absorbed in the batter.
Transfer the batter into the prepared pan. Bake for 35-40 minutes or until a wooden skewer inserted in the centre comes out clean. Do not open the oven door before the 35th minute! Let the cake cool in the pan for 10 minutes before taking it out. Let cool until room temperature before frosting.