Line a 20cm square cake tin with clingfilm and grease with the vegetable oil.
Heat half the raspberries in a microwave on high for 1 minute until piping hot and completely softened. Mash with a fork and pass through a sieve.
Add the liquid to the bowl of a stand mixer and sprinkle over the gelatine. Leave to absorb. Meanwhile, put the granulated sugar and glucose in a medium saucepan with 250ml water. Stir to combine and brush down any sugar on the sides.
Cook over a low heat until the sugar has dissolved, then turn up the heat to bring it to the boil. Using a sugar thermometer, wait for the temperature to reach 130°C. This can take at least 10 minutes, so be patient. Take the pan off the heat and leave it to cool for 1 minute.
Begin to whisk the gelatine mixture on a medium speed. Add the sugar and syrup, slowly pouring it down the side of the mixer bowl and being careful not to let it touch the whisk directly. The mixture should grow in volume like a very stiff meringue. Once you have added all the syrup, turn the speed to high and continue to whisk for another 7-8 minutes until thick and voluminous.
Working quickly, transfer half the mixture to a large bowl using an oiled rubber spatula. Briefly whisk the food colouring into the mixture in the stand mixer until evenly coloured. Alternate dollops of the mallow mixtures into the lined dish, dotting the raspberries among it. Use the end of your spoon or spatula to swirl together. Leave to set for 4 hours until firm.
Combine the icing sugar and cornflour and dust some on to a board. Turn the marshmallow out onto the board. Dip a sharp knife into hot water until hot, then carefully slice the marshmallow into 25 squares, dipping the knife back into the hot water if it gets sticky. Dust the marshmallows in the remaining icing sugar mixture to serve.