Preheat the oven to 325 degrees F (165 degrees C). Generously grease a 9-inch springform cake tin. Dust with a little sugar, and tap out the excess.
Set aside 3 tablespoons of the sugar. Place the chocolate, butter, and remaining sugar in a large, heavy-based pan. Cook over moderate heat until the chocolate and butter have melted, and the sugar has dissolved. Remove the pan from heat. Stir in the vanilla, and leave the mixture to cool slightly.
Beat the egg yolks into the chocolate mixture one at a time, beating well after each addition. Stir in the flour.
In a large bowl, scrupulously clean and grease-free, beat the egg whites until foamy. Add the cream of tartar and salt, and beat to stiff peaks. Sprinkle reserved sugar over egg whites, and beat until stiff and glossy. Use a whisk or spatula to fold 1/3 of the egg whites into the chocolate mixture, then carefully fold in the remaining whites. Carefully pour batter into the prepared tin, and tap the tin gently to release air bubbles.
Bake until well risen and a skewer inserted into the center of the cake comes out clean, for about 45 minutes to 1 hour. Check the cake after 30 minutes: if the cake appears to rise unevenly, rotate after 30 to 35 minutes. If the cake starts to crack or become too brown, place a piece of foil lightly over the top. Transfer the cake to a wire cooling rack, and remove the sides of the springform tin. Cool completely, and then remove the base. Do not attempt to remove the cake before it's completely cooled as this cake is very fragile.
