Pre heat oven to 190°c/Fan 170°/Gas Mark 5. Grease and line a 40 x 28 cm swiss roll tin, then dust with flour.
Reserving 2 tablespoons of the sugar and then whisk the rest with the egg yolks until thick and pale, next stir in the almond extract.
Whisk the egg whites in a separate bowl ensuring the bowl is free from grease as this will prevent the egg whites stiffening. Whisk until soft peaks form. Whisk in the remaining sugar little by little until you have achieved a glossy stiff mixture.
Sift half the flour over the egg yolk mixture and fold in carefully, ensuring that all the flour is incorporated into the mixture, the fold in a quarter of the egg white mixture. Sift and fold in the remaining flour, followed by the rest of the egg whites.
Spoon the mixture into the prepared tin, ensuring the mixture is spread evenly. Bake in the preheated oven for 15 minutes until lightly golden.
Sprinkle the 2 tablespoons of sugar on a piece of greaseproof paper big enough for the swiss roll to be turn out onto. Turn out swiss roll, and carefully roll up and leave to cool.
Put the plain chocolate in a heatproof bowl. Bring the cream to the boil and pour over the chocolate and stir until the chocolate has melted. Beat with an electric mixer until smooth and thick. Reserve around 1/3 of the chocolate mixture in a separate bowl and pour the rum over the remaining 2/3 of the mixture.
Unroll the cake and spread over the chocolate-rum mixture and re-roll. Cover the rolled up cake with the remaining chocolate mixture and run a fork across the top and sides to give a bark effect. Dust with icing sugar and add holly and/or a Merry Christmas plaque to decorate.