Heat oven to 350F. Grease two 9 inch round pans and just line the bottom with waxed paper. Sift together the flour and cocoa. In a large bowl, combine the eggs and sugar and beat on high until the eggs have tripled in volume will be like soft whipped cream. The eggs will flow in a ribbon when the beaters are removed. This will take some time so be prepared. Fold in the flour mixture. Then fold in the butter and vanilla. Scrape the batter into the pans and bake until a toothpick comes out clean, about 15 minutes. Let cool in pans about 10 minutes and then transfer to a rack to cool completely.