Preheat the oven to 325°F/160°C.
Line the base and sides of a 13 by 9-inch/33 by 23-cm jelly roll pan with parchment paper. Allow the paper to rise about ⅜ inch/1 cm above the sides of the pan.
To make the meringue, in a large bowl, beat the egg whites with an electric mixer until they begin to firm up. Add the superfine sugar to the whites in spoonfuls or tip into the bowl in a slow stream. Continue beating until a firm, glossy meringue forms.
Using a large metal spoon, gently fold in the vanilla, vinegar, and cornstarch. Spread the mixture inside the prepared pan and level with an offset spatula.
Bake for 30 minutes, until a crust forms and the meringue is cooked through (it will still feel soft to the touch). Remove from the oven and allow to cool in the pan.
Unmold the cooled meringue onto a fresh piece of parchment paper. Carefully peel off the lining paper.
Meanwhile, make the cream. Place the mascarpone, confectioners’ sugar, and rose water in a large bowl and whisk by hand until smooth.
Add the cream and whisk for about 4 minutes,until the cream just holds its shape. (You can do this in an electric mixer but keep a close eye on it as it’s easy to overmix.)
Spread most of the mascarpone cream over the original underside of the meringue, reserving a few tablespoons. Leave a small border around the edge of the meringue.
Scatter most of the raspberries and 1½ tablespoons of rose petals evenly over the cream.
Using the paper to assist you and starting from a long edge, roll up the meringue into a perfect log shape. Carefully transfer the log onto a serving dish. Use the remaining cream to create a rough wavy strip along the top of the log (see photo above). Chill for at least 30 minutes.
When ready to serve, dust the log with confectioners’ sugar, dot with the remaining raspberries, and scatter the remaining rose petals and pistachios evenly over the top.