Preheat oven to 350°F and lightly oil a 9 inch square pan.
Sift flours, starch and salt together and set aside.
Melt butter and chocolates in a medium saucepan over low heat, stirring frequently. Add 1 cup of sugar and stir for 30 seconds, then remove from heat and stir in vanilla.
Put eggs and remaining 1 cup of sugar into a bowl and whip until pale, thick and doubled in volume, about 4 minutes.
Fold the whipped eggs into the chocolate. When almost completely incorporated, gently fold in the dry ingredients.
Bake at 350°F for 40 - 45 minutes, until a toothpick or knife inserted into the middle comes out with very few crumbs.
