Heat the oven to 350 degrees F/180 degrees C. Grease and line with parchment paper a 10-inch/25 cm cake pan, with quite high sides.
Boil a full kettle of water and set aside. Have a roasting pan at the ready that will accommodate the cake pan comfortably.
Put the Cognac or rum, espresso, chocolate and vanilla into a saucepan and melt over a bain-marie.
Break the eggs into a large glass or metal bowl with the sugar. Set the bowl over a pot of warm water, and beat until the mixture has tripled in volume and has the consistency of whipped cream. This will take about 10 minutes.
In a separate bowl, whip the cream into peaks.
Partially fold the chocolate mixture into the egg mixture. Add the whipped heavy cream and continue folding the mixtures until evenly combined. Pour into the cake pan. Set the cake pan into the roasting pan, and slide it into the oven. Now, pour the hot water from the kettle into the roasting pan halfway up the sides of the cake pan.
Bake until set, about 1 hour. Turn off the oven, open the door and let the cake sit for 15 minutes to sink evenly back into the pan. Remove the cake from the oven and let it sit 30 minutes in the water bath. Remove from the bath and flip the cake onto a platter. Press gold leaf, if using, on the top to decorate. Serve with sweetened whipped cream on the side.