Preheat the oven to 325 degrees F. Butter and flour a 10-inch round cake pan.
Combine the butter and chocolate and melt in a double boiler over barely simmering water. Whisk together the egg yolks, salt, and all but 3 tablespoons of the sugar. Stir the melted chocolate into the egg yolks until thoroughly combined.
With an electric mixer, on medium speed, beat the egg whites until soft peaks form. Gradually beat in the remaining sugar and continue to whip until the egg whites are stiff, but not dry,
Carefully fold the chocolate mixture into the egg whites. Pour into the prepared pan. Bake for 1 hour and 15 minutes. Turn out onto a rack immediately. As the cake cools, the center will sink and crack - do not worry.
Dust the cake with powdered sugar or decorate the top with melted chocolate and serve with unsweetened whipped cream.