Preheat oven to 190'C.
Line a baking tray with grease proof paper.
Separate the eggs, putting them into two large mixing bowls. Add half the demerara sugar to the egg yolks. Whisk the sugar and yolks for about 5 minutes until the mixture is light in colour and thick enough to leave a trail when the whisk blades are removed.
Sift the flour and cocoa powder together.
Beat the egg whites with the rest of the sugar until they are stiff, then fold into the cake mixture in three batches, alternating with the sifted flour in between each addition.
Pour the mixture into the prepared tray and carefully spread to the edges using a pallet knife.
Bake for 10-12 minutes, or until the cake feels firm to the touch.
Put a large sheet of baking parchment on the work surface and sprinkle lightly with caster sugar. Turn the cake out onto the parchment and peel off the lining paper. Roll the sponge into a log while it is still warm, and leave to cool completely.
Trim about 1cm of cake from all the edges.
Place the cream and butter in a saucepan and bring to the boil.
Remove from the heat and gradually add the chocolate until you have a smooth mixture. Keep at room temperature and do not refrigerate.
Carefully unroll the cake.
Cover the inside of the cake with a thin layer of the filling allowing 2cm of one of the long sides of the cake un-iced. Roll the cake again using the paper to help you, but not rolling the paper inside.
Cut a thick diagonal slice off one end of the cake.
Transfer the larger piece of cake to a serving plate and square off the end.
Spread a little icing over the cut side of the small piece of cake and fix it to the large roll to make a branch.
Spread the remaining icing over the cake so as it is completely covered. Drag a fork or brush through the icing to create a bark effect. Dust with icing sugar before serving.
«A versatile and light sponge recipe that can be used for rolled cakes filled with jam or cream and also as a light sponge base for birthday cakes to be topped with butter icing.»