Preheat the oven to 250 C / Gas 9. Line a Swiss roll tin with baking parchment.
In a large bowl, beat the sugar and yolks until light and creamy. Add melted butter and stir in 1/2 the flour.
In another bowl, beat egg whites with 1 pinch of salt until stiff. Gently fold 1/2 of this mixture into the yolk mixture. Fold in remaining flour and remaining egg whites.
Pour cake mixture into prepared tray and smooth out to create a 1cm thick layer. Bake in preheated oven for about 7 minutes or until firm to the touch.
Remove cake from oven and immediately turn out onto a damp tea towel. Spread marmalade on the cake as soon as possible, then gently roll the cake like a Swiss roll. Let cool.
Cut a small slice off the end of the cake. Arrange on the side of the log, to represent a branch. Set aside.
In a small saucepan, melt chocolate and butter. Let cool. Spread cake with the icing. Draw veins of the bark with a fork and arrange your favourite Christmas decorations on top, if liked. Serve chilled.