Grease a 16cm x 26cm slice pan with canola oil spray. Line with baking paper, allowing the 2 long sides to overhang. Spray the paper with canola oil spray. Combine icing sugar and cornflour in a bowl. Dust one-quarter of the cornflour mixture over prepared pan.
Place the raspberries in a food processor and process until smooth. Strain through a fine sieve into a jug. Discard solids.
Place the gelatine in a small bowl. Pour in enough cold water to cover. Set aside to soften.
Combine sugar, glucose syrup and 1 cup (250ml) water in a medium saucepan over low heat. Cook, stirring and brushing the side of the pan with a wet pastry brush occasionally, for 5 mins or until sugar completely dissolves. Increase heat to medium and bring to the boil. Cook, without stirring, for 5 mins or until syrup reaches 121°C on a sugar thermometer or hard ball stage. (To test, set aside until bubbles subside. Add 1 tsp syrup to a glass of iced water – it will form a firm ball.) Remove from heat and let the bubbles subside.
Use an electric mixer to whisk the egg whites in a clean, dry heatproof bowl until soft peaks form.
Drain the gelatine and gently squeeze to remove excess water. Add to the sugar syrup in the saucepan with three-quarters of the raspberry puree. Gently swirl the pan to combine.
With the motor running, slowly pour gelatine mixture into egg white in a thin, steady stream. Continue whisking for 10 mins or until the mixture doubles in size and cools to room temperature.Add 2-3 drops food colouring and whisk to combine. Pour into the prepared pan and smooth the surface. Drizzle with the remaining raspberry puree and use a skewer to marble. Set aside for 2 hours to set.
Transfer the marshmallow to a clean work surface. Use a hot, dry knife to cut into pieces. Dust with the remaining cornflour mixture. Store the marshmallow in an airtight container at room temperature for up to 1 week.