Grease an 8 or 9-inch (20 or 23cm) square cake tin with a bit of vegetable oil. Line with baking paper and then brush the baking paper with a thin layer of vegetable oil too. Set aside.

Place the rhubarb puree into the heatproof bowl of a stand mixer with the whisk attachment fitted. Sprinkle the powdered gelatine over the rhubarb puree. Leave to sit for 5 minutes to allow the gelatine to soften.

Combine the sugar, syrup, water and salt in a large pot. Place over a medium heat on the stove and stir occasionally until the sugar has dissolved. Keep heating until the mixture reaches 114°C then remove from the heat.

Turn the mixer onto a medium-low speed and gradually stream the hot syrup into the bowl (aim for the edge of the bowl, not the whisk, to prevent splattering!). Once all the syrup has been added, increase the speed to high and leave to whisk until cooled, very thick, pale and fluffy - about 8-10 minutes.

Use an oiled spatula to scrape the mixture into the prepared pan and to spread it out into an even layer. Leave to cool and set for at least 3 hours at room temperature.

Mix equal volumes of icing sugar and cornflour in a medium bowl. Use a small sieve to dust a work surface with this mixture. Turn the marshmallows out onto this and peel away the baking paper. Dust the top of the marshmallows with more of the mixture. Use a sharp knife (brushed with oil to prevent sticking, as needed) to cut the marshmallows into squares. Dip the cut edges into more of the icing sugar mixture to prevent them sticking together.

Store in an airtight container.