1Preheat oven to 350. Butter and flour an 8inch springform pan. Sift the flour,cocoa, and baking powder into a large bowl. Beat the butter and sugar with an electric mixer at medium speed until creamy.
2Add the egg yolks, one at a time,until just blended after each addition. With mixer at low speed, gradually beat in the dry ingredients.
3With mixer at medium speed, beat the egg whites and salt until stiff peaks form. Use a large rubber spatula to fold them into the batter. Spoon the batter into prepared pan.
4Bake for 40-45 minutes. Cool cake in the pan for 5 minutes. Loosen and remove the pan sides. Invert onto a rack and remove the pan bottom. Let cool completely. Dust with the cocoa powder.
Melt the chocolate,butter and condensed milk in a double boiler over barely simmering water. Spoon the frosting into a pastry bag and pipe onto top of the cake in a decorative manner. Decorate the sides of the cake with the grated chocolate.