Brush the inside of a 1 litre charlotte (cylindrical) or ring mold with a film of vegetable oil. Invert the mold on paper towels to drain. In a heatproof mixing bowl, beat the egg yolks and sugar with a whisk, rotary or electric beater for 2 or 3 minutes or until they are pale yellow and thick enough to form a ribbon when the whisk is lifted from the bowl. Beat in the Cognac. Set the mixing bowl over a pan of barely simmering (not boiling) water, and continue beating for 3 or 4 minutes or until the mixture is foamy and hot. Then set the bowl over a pan of iced water and beat for 3 or 4 minutes longer, or until the mixture is cool again and as thick and creamy as mayonnaise. In a heavy 4 to 6 litre saucepan, melt the chocolate with the coffee over hot water, stirring constantly. When all the chocolate has dissolved, beat in the butter, one piece at a time, to make a smooth cream. Then beat the chocolate mixture into the egg yolks and sugar. In a separate bowl, with a clean whisk or beater, beat the egg whites until they are stiff enough to form stiff peaks on the wires of the whisk. Stir about quarter of the egg whites into the chocolate mixture to lighten it, then very gently fold in the remaining egg whites. Spoon the mousse into the oiled mold or dessert cups and refrigerate for at least 4 hours or until it has set. To unmold and serve, run a long, sharp knife around the sides of the mold and dip the bottom of it in hot water for a few seconds. Then wipe the outside of the mold dry, place a chilled serving plate upside down over the mold, and grasping both sides firmly, quickly turn the plate and mold over. Rap the plate on a table and the mousse should slide easily out of the mold. If the mousse doesn't unmold at once, repeat the whole process. With a wire whisk, rotary or electric beater, whip the chilled cream in a large bowl until it is firm enough to hold it's shape softly. Garnish with mousse with the whipped cream and serve.