Lightly grease Swiss roll tin and line with non-stick baking parchment, pushing it into the corners.
Preheat the oven to 180C/350F/Gas 4.
Melt the chocolate slowly in a bowl over a pan of hot water. Allow to cool slightly.
Place the sugar and egg yolks in a bowl and whisk with an electric whisk on a high speed until light and creamy.
Add the cooled chocolate and stir until evenly blended.
Whisk the egg whites in a large mixing bowl until stiff but not dry.
Stir a large spoonful of the egg whites into the chocolate mixture, mix gently and then fold in the remaining egg whites, then the cocoa.
Pour into the prepared tin and gently level the surface.
Bake in the preheated oven for about 20 minutes until firm to the touch.
Remove the roulade from the oven, leave in the tin and place a cooling rack over the top of the cake, being careful not to touch the cake.
Place a clean damp tea towel on top of the rack, and leave for several hours or overnight in a cool place - don't worry, it will sink slightly. (If the tea towel dries out, simply re-dampen it.)
Whip the cream until it just holds its shape, and stir in the orange zest.
Dust a large piece of non-stick baking parchment with icing sugar.
Turn the roulade out onto the paper and peel off the lining paper.
Trim the two long edges for a smarter finish (it also gives you a chance to taste the trimmings!).
Spread with the whipped cream and roll up like a Swiss roll: starting with one of the short edges, roll tightly to start with and use the paper to help. Don't worry if it cracks - that is quite normal and part of its charm!
Serve slices with orange segments and Grand Marnier.