In a saucepan, over medium heat, melt the caster sugar with the rhubarb pieces, orange marmalade, and orange juice. Simmer for 5 minutes until the rhubarb starts to be fully softened. Remove from the heat. Stir a few times with a whisk to break the fibres if necessary or pulse once or twice in a food blender. Set aside to cool.
Meanwhile, prepare the pavlova. Preheat oven to 120C and Line a baking tray with parchment paper. Drawn the size of the pavlova with a pencil on the paper for guidance.
Place the egg whites into a spotlessly clean mixing bowl of food mixer and beat on full speed until stiffly beaten. Turn the speed of the mixer down and slowly add the sugar, adding just a little at a time.
When all the sugar has been incorporated, add the cornflour and vinegar, and give one final whisk on high speed until glossy and very stiff.
Pipe the meringue mixture on the prepared baking tray.
Bake in the oven for 1 hour until the meringue mixture is very firm to the touch but still soft in the middle. I normally leave mine to cool in the oven with the door ajar.
Remove from the oven and set aside until fully cooled.
Once ready to serve., in a large bowl, whip the cream until soft peak. Fold in the rhubarb compote to create a rhubarb fool. Transfer the mixture on the pavlova and decorate with extra compote, a few edible flours and mint leaves if desired.