Start with the rhubarb paste. Place the rhubarb, sugar and water into a small saucepan. Cover and heat on a medium heat until the rhubarb breaks down. It should take about 5-6 minutes. Turn off the heat.
Use a hand blender to blend the rhubarb in the pan, then add in the gelatin and food colouring and heat slowly, whilst stirring until the gelatin is completely dissolved. Turn off the heat and leave to cool.
Next, prepare the gelatin: If you’re using sheet gelatin, place it in a small pan with the water and vanilla extract. Heat slowly for 5 minutes, stirring regularly, until the gelating has dissolved. Then take off the heat and pour it into the bowl of your mixer. OR If you’re using powdered gelatin, pour it straight into the bowl of your mixer and add the water and vanilla extract. Whisk with a fork until the gelatin has dissolved and the mixture has thickened.
Take a medium sized saucepan, place a candy thermometer in. Pour in the water, and then the sugar, corn syrup, and salt. Do not stir. Heat the mixture on a medium-to-high heat until boiling. If you notice any sugar crystals on the side of the pan, you can dip a pastry brush in water and brush down the sides of the pan. This will prevent the sugar crystals falling into the mixture, causing it to crystallise. Continue to heat the mixture until it reaches 119c/247f – 121c/250f (still no stirring!). Once it reaches this temperature, turn off the heat, and carefully remove the thermometer.
Go back to your mixer, and start whisking the gelatin mix at a medium speed using the balloon whisk attachment. Whilst whisking, very slowly (and carefully) pour the sugar mixture into the bowl (down the side of the mixing bowl – so it doesn’t splash you) in a thin stream. Once all of the sugar mixture is in, cover the mixer in a clean kitchen towel and turn up the speed to high. Whisk for 10 full minutes (you can take the towel off after a couple of minutes – once the marshmallow mix has started to thicken)
Whilst the mixer is on, prepare your baking tin. Take a tin approx. 9”x13” and spray with cooking oil. Wipe it around with a kitchen towel, so all the corners have a thin coating of oil. Put to one side.
Once the 10 minutes is up, take a stiff spatula and immediately scrape the marshmallow mix into the prepared tray. It will be very sticky! Once it’s all in, spray your hands with cooking oil, and flatten the marshmallow down and push to the edges.
Working quickly, dot your cooled rhubarb paste on top of the marshmallows. Take a cocktail stick and swirl it around – so there are no large pools of rhubarb (just gentle swirls).
Leave the marshmallows to set – uncovered – for at least 4 hours (up to 24 hours)
After the marshmallows have set, mix the confectioners/powdered sugar and cornflour in a small bowl. Sprinkle a couple of tablespoons of this mix on top of the marshmallow, and tip the marshmallow out onto a cutting board. Sprinkle again with more of the cornflour mix and cut the marshmallow into chunks using a sharp knife or a pizza cutter. It’s a good idea to dip your knife in the cornflour mix after each cut to stop the marshmallow sticking to the knife. Also, after each cut, move the cut pieces apart (or they’ll meld back together again). Once the marshmallow has been chopped into chunks, toss each chunk in the cornflour mix to cover all sides.
Serve straightaway or place in an airtight box until ready to serve.