Preheat the oven to 180 degrees C and grease a 23cm springform cake tin.
Melt the chocolate in a double boiler, stirring until smooth. You can also use the microwave.
Using an electric mixer, beat the egg whites with the salt until foamy. Add cream of tartar and beat to the soft peak stage. Add sugar, one tablespoon at a time, and beat to stiff peaks.
Sift in the cornflour and fold in. Whisk in the eggs one at a time. Add vanilla and beat. Fold in the chocolate.
Pour the batter into the cake tin and bake 40 minutes. Cool in the tin.