Hydrate the gelatine powder in 50 ml of cold water. If using leaves, soak them in a large volume of cold water for at least 5 minutes.
Prepare a dish, grease it with a little neutral oil and cover it with parchment paper.
In a saucepan heat the sugar, golden syrup and 150 ml of water. When the mixture comes to a boil, remove it from heat. Add the gelatine and mix well until it is dissolved. If you are using the gelatine sheets, drain them well before adding them.
Pour the mixture into a bowl. Whisk until the mixture has a soft meringue consistency. It will take a little while to get the right consistency, about 10 minutes. Then pour into the dish you have previously prepared
Sprinkle with a little icing sugar/cornstarch mixture and let the marshmallow dry overnight, uncovered
The next day the marshmallow is ready and you can cut it into small cubes. If the knife is a bit sticky, you can grease it lightly with neutral oil. Once your marshmallows are cut, coat them well with the icing sugar/starch mixture.
You can keep them for a few days at room temperature, but I doubt they will last long ;)