For the dusting mixture
For the marshmallows
To make the dusting mixture, mix together the icing sugar and cornstarch. Grease a large 22cm square baking tin with cooking spray or a thin layer of oil and dust the icing sugar mixture over – just enough to cover the surface.
To make the marshmallows, in the bowl of a stand mixer fitted with a whisk attachment, combine the water and gelatine. Stir and allow to bloom for 5 minutes.
In a small pot, combine the sugar, glucose syrup, water and salt. Place over a medium heat and bring to a boil. Boil until the sugar syrup reaches 120°C or soft-ball stage. Take off the heat immediately.
Turn the mixer on and slowly pour the syrup in a steady stream down the inside of the bowl. Once all the syrup has been incorporated, leave to mix for a further 20 seconds.
Increase the speed to high and whisk for another 8 minutes until thick. Add the rose water and advieh and immediately pour into the baking tin. Allow to set overnight.
Dust a surface with the icing sugar mixture and turn the marshmallows out. Grease a knife with oil and dust it with the icing sugar mixture and cut the marshmallows into 5cm blocks. Dust the cut sides to prevent them from sticking together.
Sprinkle the advieh and dried rose petals over the marshmallows. Store in an airtight container for up to 4 weeks.