You will need a 33cm x 24cm Swiss roll tin and an electric mixer.
1. Preheat the oven to 160C/140C fan/273F/ gas 3. Line the base and sides of a 33cm x 24cm Swiss roll tin with grease-proof paper. Allow the paper to come about 1cm above the sides of the tin.
2. In the bowl of a standing mixer fitted with balloon whisk, or in a large, clean bowl using a hand mixer, beat the egg whites until soft peaks form. Start adding the sugar by a spoonful whilst beating continuously.
3. When there’s only a little sugar left, stir the cornflour into it, turn the speed of the mixer down and add to the meringue. Beat in the vanilla and vinegar on low speed, then turn it up and beat for a couple more minutes until the meringue is stiff, glossy, and the sugar has almost completely dissolved.
4. Tip the meringue onto the prepared lined tin and spread it evenly with a spatula or palette knife; you want it smooth as it will bake as is – it won’t spread any more.
5. Transfer it into the oven and bake for 30 minutes until the surface is crusty but not coloured. remove from the oven and cool in the tin.
6. To make the filling, place the mascarpone, sugar and rose water in a bowl and beat briefly to loosen it up. Add the cream and beat with a hand mixer or whisk until it forms soft peaks; don’t overmix or it will be too stiff to spread.
7. Now for the fun bit: when the meringue is cold, dust it lightly with icing sugar. Prepare a large piece of fresh parchment, cover the top of the meringue, then place a flat tray or board over it and flip it over. Peel the parchment off the meringue bottom carefully.
8. Spread most of the cream filling over the meringue, leaving an edge close to you along the long side. Scatter the raspberries over the cream, followed by most rose petals and pistachios.
9. Fold the long edge close to you over to start off the rolling, then use the paper to roll the rest into a log. Don't worry if it cracks - it will end up quite surprisingly flexible, within reason. Shape it gently up with your hands.
10. Carefully transfer the log on to a serving dish – just trim the paper around the log leaving only the piece underneath it. Chill for at least 30 minutes before serving.
11. When ready to serve, dust the log with icing sugar, scatter the remaining rose petals and pistachios over the top, and serve with the remaining raspberries on the side. Slice with sharp, non-serrated knife.