1. Preheat oven to 350 degrees. Grease a 10x15-inch jelly roll pan. Line the pan with parchment paper and grease the paper. 2. In the bowl of an electric mixer, beat the egg yolks and sugar together until the mixture is pale and fluffy. 3. Add the cocoa to the batter. 4. In a separate bowl, beat the egg whites until stiff and gently fold them into the batter. 5. Pour the batter into the prepared jelly roll pan. Bake for 20 minutes. Remove from the oven and turn the cake over onto a clean linen dish towel that has been dusted with confectioners sugar. Lift off the pan and let the cake cool for 10 minutes. Peel off the paper from the cake. Carefully roll the cake with the towel, jelly roll fashion, and let the cake remain in this position on a rack for 20-30 minutes while it cools completely. 6. When the cake is cooled, unroll to fill (IMPORTANT* If the cake is even slightly warm, the whipped cream filling will weep.) Spoon a generous amount of the whipped cream onto the inside of the cake, smoothing it to within 1-inch of all the edges. 7. Roll the cake up to form a log. Place it seam side down on a platter. Refrigerate the cake roll until serving time.