Cooking oil spray, for greasing
300g packet frozen raspberries, defrosted
9 gelatine leaves
550g caster sugar
80g eggwhites (from 2-3 eggs), at room temperature
Pinch of fine salt
5 Tbsp pure icing sugar
3 Tbsp cornflour
1 Tbsp citric acid
Note: Allow 4 hours standing
Lightly grease a 23 x 23mm square cake tin with cooking oil spray. Line base and sides with baking paper.
To make dusting sugar, put icing sugar, cornflour and citric acid in a bowl. Mix to combine. Dust a small amount over base of prepared tin.
Put raspberries in a food processor and blend. Strain through a fine mesh sieve into a large bowl.
Put gelatine in a bowl and cover with cold water. Set aside for a few minutes to soften, then squeeze out excess liquid. Add leaves to raspberry juice.
Put caster sugar and water in a medium saucepan over a low heat. Cook, stirring, until sugar dissolves. Increase heat to medium and cook for a further 5–10 minutes or until syrup thickens and reaches 125ºC on a sugar thermometer. Or, drop a tiny amount onto a saucer of cold water. If syrup hardens immediately, it is ready.
Add sugar syrup to raspberry mixture and stir to dissolve gelatine.
Put eggwhites and salt in the bowl of an electric mixer and beat until soft peaks form. Gradually add hot raspberry mixture in a steady stream, beating continuously on medium until mixture doubles in size and becomes thick and glossy.
Pour marshmallow mixture into prepared pan and smooth top with a spatula. Stand at room temperature for 3-4 hours or until firm.
Turn marshmallow out onto a board and use a sharp knife to cut into squares. Roll squares in remaining dusting sugar. Serve or store in an airtight container in a dry place for up to 1 week.