Preheat oven to 350 degrees F (175 degrees C). Line a 10x15 inch jelly roll pan with parchment paper. Stir together the flour, cocoa, baking powder, and salt; set aside.

In a large bowl, beat egg yolks until thick and pale yellow. Fold in flour mixture. In a large glass or metal mixing bowl, beat 4 egg whites until foamy. Gradually add white sugar, continuing to beat until stiff peaks form. beat in vanilla. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pan.

Bake in preheated oven for 12 to 14 minutes, or until cake springs back when lightly tapped. Turn cake out onto a towel dusted with confectioners' sugar, and roll up until cool.

To Make Frosting: In a small saucepan, stir together brown sugar and water. Cook, stirring constantly, until sugar spins a thread when dropped from a spoon. In a large bowl, beat together remaining 2 egg whites until foamy. Slowly pour brown sugar liquid into egg whites while beating. Beat until frosting forms stiff peaks. Unroll cake and apply frosting in an even layer. Roll cake back up and refrigerate.