In a medium bowl, stir together the 1/2 cup cocoa powder, the flour, and baking powder. Stir in chocolate-egg yolk mixture until smooth. In a large bowl, beat egg whites with an electric mixer on medium speed until stiff peaks form (tips stand straight). Stir a small amount of the beaten egg whites into the chocolate mixture to lighten. Fold chocolate mixture into remaining egg whites. Spread in the prepared pan.