For the roulade, preheat the oven to 180C/465F/Gas 4.
Lightly grease a 23cm x 33cm/9in x 13in Swiss roll tin with a little sunflower oil and line with baking parchment.
Place the egg yolks and sugar into a large bowl and whisk until light and pale.
Sift the cocoa powder into the egg yolk mixture and whisk until completely combined.
In a clean bowl, whisk the egg whites until stiff peaks form when the whisk is removed.
Add half of the whisked egg whites to the cocoa mixture and mix together well, then gently fold in the remaining whisked egg whites until just combined.
Pour the mixture into the Swiss roll tin, moving the tin so that the mixture spreads evenly into the corners.
Transfer to the oven to bake for 20 minutes, or until firm to the touch, then remove from the oven. Place a clean, dry tea towel on top of the roulade and on top of this place another tea towel that has been soaked in cold water and well wrung out. Leave in a cool place for an hour.
For the filling, heat a dry frying pan until hot and add the almonds and hazelnuts and toast until golden-brown all over.
Remove the nuts from the pan, allow to cool slightly, then roughly chop.
Add the sugar to the same pan and return to the heat. Cook until the sugar has melted and caramelised to a golden-brown colour.
Add the chopped nuts to the caramel and swirl to combine and coat the nuts in caramel.
Pour the caramel and nut mixture into a well-greased, lined baking tray and leave to cool and set. Once set, remove the mixture and break up into small pieces.
Whisk the cream until soft peaks are formed when the whisk is removed, then fold the caramel nut pieces into the cream.
To serve, remove the tea towels from the sponge, and turn out onto a piece of baking parchment that has been dusted with sifted cocoa powder.
Peel the lining paper from the sponge and spread the whipped cream mixture evenly over the top.
Starting from a long end, roll up the sponge, using the cocoa-dusted paper to help lift the cake as you roll it forward.
Once rolled up, dust the roulade with more sifted cocoa powder and sprinkle the grated chocolate over the top.
