You can make the chocolate filling well ahead of time. To do this, break the pieces of chocolate into a basin and add 2 tablespoons of water. Now place the basin over a saucepan of barely simmering water, making quite sure the basin isn't actually touching the water. Keep the heat at a minimum and wait for the chocolate to melt. Then remove from the heat and beat with a wooden spoon until smooth.
Next beat the egg yolks – first on their own and then into the warm chocolate mixture. Then as soon as the mixture has cooled, whisk the egg whites to the soft-peak stage and gently cut and fold them into the chocolate mixture. Cover the bowl with clingfilm and leave it in the refrigerator till you're ready to use it, but for a minimum of an hour.
Drain the cherries in a sieve. Discard the syrup, then place the cherries in a shallow dish, spoon over the cherry brandy and leave aside till needed. Pre-heat the oven to gas mark 4, 350°F (180°C).
For the base, first place the egg yolks in a bowl and whisk them with an electric hand whisk until they begin to thicken. Then add the caster sugar and continue to whisk – but be careful not to overdo this as it can eventually become too thick, which makes it difficult to fold in the egg whites. Now fold in the cocoa powder. Then, using a spanking clean bowl, whisk the egg whites to the soft-peak stage. Then take 1 large spoonful and fold it into the chocolatey mixture to slacken it, and then gently cut and fold in the rest of the egg whites.
Now pour the mixture into the prepared tin and bake the cake on the middle shelf of the oven for about 20-25 minutes or until it's springy in the centre. It will look very puffy, but a little finger gently pressed into the centre should reveal that it is cooked. It's important not to overcook it. Remove it from the oven and don't panic as it sinks down, because this is quite normal. Leave until it's absolutely cold, then turn it out on to a sheet of greaseproof paper which has been lightly dusted with sieved cocoa powder. Then carefully peel away the silicone paper.
Drain the cherries again in a sieve placed over a bowl, to catch the liqueur, and sprinkle all but 1 tablespoon of the liqueur all over the base of the cake. Next remove the chocolate filling from the fridge and, using a palette knife, spread it carefully and evenly all over the surface of the cake. Next softly whip the double cream and spread this all over the chocolate filling, then lightly press the cherries into the cream.
Rolling this cake up is going to be a lot easier than you think. All you do is take hold of one edge of the greaseproof paper beneath it and lift it – as you lift, the cake will begin to come up. Just gently roll it over, pulling the paper away as it rolls. If the cake cracks as you roll it, this is not a problem – this can look very attractive and, anyway, it's all going to get covered in chocolate!
To make the chocolate curls (don't worry – it's much easier than it sounds), all you do is melt the chocolate as before, taking great care not to overheat it, then pour it on to an upturned plate, 6 inches (15 cm) in diameter. Then place in the fridge for about 45 minutes until it's set. The chocolate should be firm when you touch it; if it's too soft, it won't make nice curls.
To make the curls, use a cheese slicer (available from kitchen shops), or a very sharp knife will do if you hold the blade with both hands. Start at one end and just pull the slicer or knife towards you along the surface of the chocolate until curls form. As you make the curls, place them in a plastic container in the fridge, as they're much easier to handle later on if they're well chilled.
Now you can decorate the cake. Spoon the cherry jam into a small saucepan, adding the reserved tablespoon of liqueur from the cherries, warm gently, then brush it all over the surface. Place the chocolate curls all over that and finally, sift a little cocoa powder to lightly dust the surface.
Note: This recipe contains raw eggs.