Preheat the oven to 180oC / fan 160oC / gas mark 4 and line a Swiss roll tin with baking parchment. Separate the eggs and beat the whites in a bowl until they form soft peaks. Pour in 60g of the sugar a spoonful at a time and continue beating to form stiff peaks.
In a separate bowl, whisk the yolks with the remaining sugar until smooth and creamy, then fold this into the whisked egg white. Mix together the flours, cocoa powder and baking powder and fold into the egg mixture. Spread out onto the lined tray and bake for 10 minutes.
While the sponge is baking, cut two sheets of baking parchment, each about 10cm longer than the baking tray. Lay both out on a clear, flat surface and sprinkle some caster sugar on them. Once the sponge is cooked but still warm, turn it out onto one of the sheets of parchment and peel off the baked parchment. Tip the sponge onto the other sheet of parchment so that you end up with the top side of your sponge showing. Tightly roll your sponge up into the bottom layer of parchment to make a rolled sausage of sponge. Leave to cool.
Whip 250ml of the double cream until it forms soft peaks. Put the chocolate spread in the microwave for 15-20 seconds to soften, then unroll the sponge and spread the chocolate spread followed by the cream. Roll up the sponge and lay on some baking parchment.
Toast off the chopped hazelnuts in a dry frying pan until they begin to smell nutty (don’t burn them) then set aside to cool.
Heat the remaining 150ml of cream in a saucepan over a low heat until just before simmering. Break the chocolate into a bowl and once the cream is hot, pour this over the chocolate and stir together to melt the chocolate. Once the chocolate and cream have melted together, spread evenly over the rolled cake and sprinkle with the hazelnuts. Put in the fridge for half an hour for the chocolate to set and you’re done!