Beat egg yolks until thick. Add sugar. Beat thoroughly. Add cocoa, beat thoroughly. In a separate bowl beat whites until stiff but not dry. Fold ¼ of beaten whites into chocolate mixture to lighten it, then fold in the remainder. Spread on jelly-roll pan covered with waxed paper or baking parchment. (Do not grease pan or paper!)
Bake 15 minutes at 350 degrees F. Turn out at once on lightly floured tea towel. Remove paper quickly and roll the cake lengthwise, rolling the towel in as you go. Allow to cool, then unroll and spread with the sweetened whipped cream. Reroll and chill before slicing.