Preheat the oven to 325 degrees F (165 degrees C). Line a 10x15 inch jellyroll pan with parchment paper. Sift together the cocoa, flour and baking powder; set aside.
In a medium bowl, whip the egg yolks and half of the sugar with an electric mixer until thick and pale. Set aside. In a separate larger bowl, combine the egg whites and vanilla. Whip with clean beaters until foamy. Gradually sprinkle in the remaining sugar while continuing to whip until stiff but not blocky. Fold the yolks into the whites by hand, then fold in the dry ingredients. Spread evenly in the prepared pan.
Bake for 15 minutes in the preheated oven, until the cake springs back when lightly pressed. Try not to let it bake too long or it will be difficult to roll. When the cake is done, run a knife around the edge to loosen and turn out onto a sheet of parchment paper or aluminum foil. Sprinkle confectioners' sugar on both sides and allow to cool.
Spread whipped cream on one side and place the bananas along the length. Roll the cake up around the bananas. Place on a serving platter with the seam side down. Frost with chocolate frosting.
