Preheat oven to 350 degrees.
Butter and flour a 10 inch springform pan.
Line the bottom with a round of wax paper. Set aside.
Melt the chocolates and butter together in a saucepan over low heat, stirring well to blend.
Set aside to cool.
Beat the egg yolks with the sugar until thick and lemon yellow in color, about 5 minutes.
Stir in vanilla.
Whisk the egg whites with the salt until stiff. Set aside.
Gently fold the chocolate mixture into the yolks, then fold in one third of the egg whites.
Fold in remaining whites until no streaks of the white remain.
Pour batter into the prepared pan and bake in the center of the oven for 35-45 minutes or until toothpick inserted in center comes out clean.
The cake will rise a great deal during baking.
Cool the cake in the pan for 10 minutes, then remove sides of pan.
Invert the cake onto a rack and remove the bottom, leaving the wax paper still on cake.
Let the cake cool completely. It will fall considerably.
Remove the wax paper before glazing.