Preheat the oven to 180°C (fan oven 160°C), gas mark 4. Line a 28 x 38cm (11 x 15in) Swiss roll tin with non-stick baking parchment, making sure the top edge is about 2.5cm (1in) above the level of the tin.
Put the chocolate in a small heatproof bowl with 150ml (1/4 pint) water and melt slowly over a pan of gently simmering water. Stir until smooth, then leave to cool slightly.
Whisk the egg yolks and caster sugar together using an electric whisk (mixer or hand-held) until very thick and pale. Add the cooled chocolate and stir until evenly blended. Whisk the egg whites until they form soft peaks.
Fold about one-third of the egg whites into the chocolate mixture using a large metal spoon. Fold in the remaining egg whites. Pour into the prepared tin and bake in the preheated oven for 20-25 mins or until just firm. Put a wire rack over the tin and cover with a damp tea towel. Leave overnight.
To make the filling, whip the cream until it just holds its shape. In a separate bowl, stir the orange zest and 1 tbsp of juice into the conserve.
Dust a large piece of non-stick baking parchment with icing sugar. Flip the roulade, cooked side down, onto the parchment and peel off the lining paper.
Spread the roulade with the whipped cream, then dot evenly with the conserve. Roll up the roulade, keeping it as tight as possible, and pull away the paper. Cover lightly with cling film and keep chilled until needed.
Decorate with chocolate curls and serve in slices with chilled single cream.
The roulade also has an optional chocolate coating that looks impressive but is very simple to do. Cut out a sheet of baking parchment the same diameter and length as the rolled roulade. Put the roulade in the freezer for 30 mins. Brush the parchment all over with a thin layer of melted chocolate, about 100g (3.5oz). Remove the roulade from the freezer and drape the paper over the roulade, melted chocolate side down. Press down evenly all over. Return to the freezer for 20 mins, then gently peel away the paper and you will be left with a crisp chocolate covering. Keep in the fridge and dust lightly with cocoa powder before serving.