Beat the sugar, egg yolks, water and orange zest in a bowl using the whisk on a hand mixer for approx. 5 mins. until the mixture becomes lighter in colour. Beat the egg whites with the salt until stiff. Combine the flour, cocoa powder and baking powder, sieve into the mixture in layers along with the egg whites, carefully fold in using a rubber spatula. Spread the sponge mixture into a rectangle (approx. 5 mm thick) on a baking tray lined with baking paper.
Approx. 8 mins. in the centre of an oven preheated to 200°C. Remove the sponge, tip the cake and paper out onto a fresh sheet of baking paper, wipe the upper layer of paper with a damp towel and peel away carefully. Cover the sponge immediately with the upturned tray, leave to cool.
Combine the cream cheese with the sugar and reserved orange zest. Carefully fold in the whipped cream. Spread the cream cheese mixture on top of the sponge, leaving a border of approx. 2 cm all the way around, roll up tightly from the long end.
Place the chocolate in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth, allow to cool slightly. Beat the butter for approx. 2 mins. using the whisk on a hand mixer. Stir in the icing sugar and chocolate, fold in the whipped cream, cover and chill for approx. 30 mins. Spread the chocolate mixture all over the roulade. Score the surface with a fork to create a bark effect. Cut off one end of the roulade and press it into the side of the cake as a branch. Cover and refrigerate for approx. 1 hr. Dust with icing sugar.