Pre-heat oven to 350F (180C). Grease and flour an 8 inch (20cm) springform pan or pie plate.
Melt the chocolate and the butter either in the microwave or bain-marie**, Let cool.
In a medium bowl beat the egg whites until stiff peaks appear. Set aside.
In a large bowl beat sugar and egg yolks until light and fluffy, approximately 5 minutes. Add the cooled chocolate mixture and combine on low speed. Add the flour, salt and vanilla, beat to combine. Gently fold in the egg whites and combine well.
Pour the batter into the prepared pan and bake for approximately 25-30 minutes. The cake is done when a crust is formed on top and it starts to crack ( you an also insert a toothpick into the centre and when it comes out clean it is done - do not over bake).
Let the cake cool completely, then dust with some powdered / icing sugar before serving.
**Place some water in a small pot over medium low heat, place a heatproof bowl on top and add the butter and chocolate heat (stirring occasionally until melted).