1. Preheat the oven to 180°C, gas mark 4. Grease a 33 x 23cm Swiss roll tin and line with parchment paper then lightly grease the paper. Place the hazelnuts in a small roasting tin and toast in the oven for 4-5 minutes until golden. Leave to cool then whizz to fine crumbs in a food processor.
2. For the filling, beat together the mascarpone, 20g of icing sugar and the vanilla extract. Cover and chill until needed.
3. Place the yolks in a large bowl with the caster sugar and beat using a hand-held electric whisk on high speed for about 2-3 minutes, until pale and thickened. Sieve over the flour and fold in along with the toasted hazelnuts.
4. Using clean beaters whisk the egg whites in another bowl until forming soft peaks then gradually and gently fold into the egg yolk mixture. Pour into the prepared tin and bake for 15 minutes until golden and springy to the touch.
5. Place a large sheet of parchment paper on the work surface and sprinkle with a little caster sugar. When cooked, carefully up-turn the sponge onto the paper and remove the parchment from the base of the sponge. Leave to cool completely. Spread with the mascarpone filling and sprinkle over the fruit. Using the sugar-dusted paper to help, roll up the sponge tightly, from the long side. Place on a serving board or platter join side down and dust with the remaining icing sugar. Cut into thick slices to serve.