Preheat oven to 350 degrees. Oil an 11 by 16 jelly roll pan. Line with waxed paper and oil the paper.
Beat egg yolks until light and fluffy and gradually add sugar. Continue beating until mixture is thick and pale in color.
Melt chocolate with coffee in saucepan over low heat and cool. Add to egg mixture. Beat egg whites into stiff peaks. Fold into mixture.
Spread batter evenly on baking sheet and bake for 15 to 20 mintues, until toothpick comes out dry. Leave out of oven for 10 minutes, then turn onto waxed paper (overlap 2 sheets if necessary to obtain 18 inch width). Let cool.
Remove waxed paper. If edges of cake are very dry, cut off edges with sharp knife. Dust top of cake with cocoa.
To make Filling: Beat cream with vanilla and sugar until peaks are slightly more than soft. Set aside 14 of whipped cream, and spread remainder on cake leaving about 1 inch of narrow side uncovered. Starting from opposite narrow side, roll up like jelly roll.
Frost top with remaining whipped cream. If desired, decorate with chocolate curls. Refrigerate or freeze.