Mix the icing sugar and cornflour together. This is your coating and what will stop the marshmallows sticking together. Set aside
Combine the gelatin, cold water and vanilla in the bowl of a stand mixture and whisk with a fork until it thickens to a paste
Meanwhile put the corn syrup, sugar, salt and second amount of water in a saucepan and stir briefly off the heat
Bring the mixture slowly to the boil. Don't stir!
Uncover and place a candy thermometer in the pan. Bring to soft ball stage - 240F /115C, no stirring. If you don't have a thermometer, keep a glass of ice cold water near the pan. When the mixture comes to a full boil, scoop a quarter teaspoon of it and drop it in the water. If you can form a squishy ball in the water, it's ready
Start the mixer with the bloomed gelatin in it going on a medium speed
Now the fun part...You need to slowly and carefully start to pour the hot syrup mixture into the mixer. Be really careful here, you don't want to end up with a syrup burn trust me.... I had one and I was sat with my hand in a jug of cold water for 10 hours before the heat started to come out of it....go slow and aim for the side of the bowl rather than straight into the gelatin
Once all the syrup is in, turn the mixer to high and throw a clean tea towel over it to avoid the mixture going everywhere. In 15 or 20 minutes (depends on your mixer...) you should see peaks starting to form like you'd see in meringue mixture
While you're waiting, line a 13cm x 9cm tin or glass dish with parchment paper and add a good dusting of the icing sugar mixture on top of it, edge to edge. This will make the paper peel of more easily when the time comes
When the marshmallow mixture is ready, scrape it into the lined tin and smooth off the top with wet hands or the back of a wet spoon
You now need to let it cool, uncovered, at room temperature overnight. Patience required!
In the morning, dust the top of the marshmallow block with some more icing sugar /cornflour and turn out onto a dusted chopping board. Peel off the paper, it should come away quite easily
Now cut the block into equal sized squares. I find kitchen shears are the easiest way but you can use a pizza cutter or sharp knife
You can also use cookie cutters to cut the shapes out such as hearts or stars but then you're left with the outline!
Toss the cut marshmallows in the bowl of icing sugar so they don't all stick together and store in a lidded container in until they're gone
You can use agar (vegan gelatin) here BUT it works a bit differently. Powdered agar is a straight substitution for powdered gelatinin but you need to boil it to activate. So put the agar and cold water in a pan instead of in the mixer bowl, let it sit 5 minutes then start to heat up, bringing to the boil then letting it go for 5 minutes. Pour the mixture into the mixer bowl and start it on a medium speed, then pour your syrup mixture in as directed above