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Method

1. Preheat the oven to 180C/350F/Gas 4 and lightly grease and line a Swiss roll tin with non-stick baking paper, taking care to push it into the corners.

2. Gently melt the chocolate slowly in a heatproof bowl set over a pan of gently simmering (do not alllow the water to touch the bowl). Once melted, set aside to cool slightly.

3. Place the sugar and egg yolks in a separate bowl and whisk until light and creamy (an electric hand whisk on a high speed is the easiest way to do this). Add the cooled chocolate and stir until well combined.

4. Whisk the egg whites in a large mixing bowl until stiff peaks form when the whisk is removed. Stir a large spoonful of the egg whites into the chocolate mixture, mix gently and then fold in the remaining egg whites.

5. Fold in the cocoa powder then pour into the prepared tin and gently level the surface.

6. Bake in the preheated oven for about 20 minutes or 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 (if the tea towel dries out, re-dampen it). Don't worry if the cake sinks slightly.

7. Whip the cream until it just holds its shape when the whisk is removed, then stir in the orange zest.

8. Dust a large piece of baking paper 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: 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!

9. If using, soak the orange slices or segments in Grand Marnier, then remove any excess liquid and use them to decorate the roulade.