Beat 4 of the egg yolks with 150g sugar until you get a smooth mixture

Melt 50g of the butter and add to the mixture along with the vanilla sugar

Beat the egg whites until stiff and then add to the mixture with a pinch of salt

Pour the mixture into a rectangular baking tin lined with lightly buttered grease proof paper

Cook in a pre-heated oven (220 degrees) for 10 minutes

Remove from the oven and place on a clean tea towel carefully removing the sponge from the baking tray and removing the grease proof paper slowly

Form a roll with the sponge and once it has cooled sufficiently place in the fridge for an hour

Prepare the filling using 1 egg yolk, 50g of butter, the chestnut puré, the rum (or brandy), the cocoa powder and the rest of the sugar - mix everything together until it forms a smooth paste

Remove the sponge from the fridge and unfold carefully then smooth over the paste and form into a roll again before placing in the fridge for a further hour

Slowly melt the chocolate in a bowl suspended over simmering water in a pan (make sure the bowl isn´t touching the water) then once it has cooled a little remove the roll from the fridgerr and pour the chocolate over the roll - return to the fridge for at least an hour before serving

Tips: You can make vanilla sugar by placing a vanilla pod in a jar with sugar and leaving it for 24 hours (or longer) alternatively you could use sugar with a couple of drops of vanilla essence.