Prepare a 12″ x 15″ tray and lay out oiled baking paper on it. Melt the chocolate with the coffee in a container on low heat and let it cool.
Whisk the egg yolks with the sugar until whitened. Mix them with the chocolate and add the cognac. Beat the egg whites to fluffy snow and stir them in carefully to the chocolate blend.
Pour the blend into the tray and smooth it out. Bake for no more than 15 min. in a preheated 338°F (170 °C) oven. Check readiness with a toothpick, it needs to come out dry. Take out the ready layer, cover it with a moist towel and leave in the fridge for 1 hour.
Sprinkle cocoa on baking paper that is a little larger than the tray. Invert the layer with the tray carefully on top and remove its baking paper. Beat the cream with the vanilla and smear the layer. Wrap into a roll by lifting the paper from one end.
If the roll cracks, you can fill it with cocoa or cream.