Fold one third of beaten whites into batter. Fold in remaining whites. 6 Dust a clean cloth with confectioners' sugar. Turn cake out onto prepared cloth. Remove waxed paper.
Trim cake edges.
Starting with a long side, roll up cake, jelly-roll style. Transfer, seam-side down, to a wire rack to cool for 30 minutes. Spread whipped cream over cake to within 1 inch of edges. Place, seam-side down, on serving plate. To prepare frosting, in a small saucepan, bring heavy cream, melted chocolate, and butter to a boil over medium heat, stirring vigorously until blended.