Using a whisk or in an electric mixer, blend sweet potatoes and butter or margarine. Add egg yolks, sugar and melted chocolate. In another clean bowl, with clean, dry beaters, whip the egg whites with the salt just to break up and foam whites slightly. Fold one third of egg whites into chocolate mixture and work in well to loosen. Then, gently fold in remaining two-thirds of egg whites in two installments, blending well but taking care not to deflate the mixture.
Cake rises somewhat and looks dry and slightly cracked on top when done.
Middle should be soft but firm. Cool in pan for 20 minutes, then remove to a wire rack.