Preheat oven to 350°F. Line greased 10" x 15" jelly roll pan with wax paper.
Make the cake Beat egg whites in a medium bowl until soft peaks form. Add cream of tartar. Beat until stiff. Using the same beaters and a larger bowl, beat egg yolks, sugar, cocoa, vanilla and salt until frothy. Carefully fold in egg whites. Spread in prepared pan. Bake at 350°F for 15-20 minutes until a toothpick poked in the centre comes out clean. Sift cocoa over a tea towel to lightly dust it. Turn cake out onto cocoa-coated towel. Peel off wax paper. Begin at the shorter side and roll both the towel and cake together, continuing to dust with cocoa as you go. Let cake cool completely in this form.
Filling and assembly Beat cream, cocoa, icing sugar (and liqueur, if desired) in a medium bowl until thick. Carefully unroll the cake. It may break a little, but this is OK since it can be put back together as you roll it back up. Spread 1/2 the filling on the cake and gently roll back up without the towel. Spread the outside of the cake with remaining filling and garnish, if desired.