Place the sugar, liquid glucose and water in a saucepan, set over medium heat and stir to wet the sugar. Once the sugar has dissolved, stop stirring as it comes to the boil. Too much stirring or splashing can result in crystallised sugar. Cover the saucepan with a lid and leave the sugar syrup to cook for 2 minutes for the steam to dissolve any excess sugar crystals attached to the sides of the pan. If there are still some sugar crystals, return the lid to the pan for another minute, or brush the sugar crystals away with a wet pastry brush. Just one crystal can crystallise all of the syrup!
While the syrup is cooking, combine the gelatin powder and cold water in a large mixing bowl, stir to combine and set aside for 10 minutes for the gelatin to bloom (set).
Optional - if you'd like to add egg whites to your marshmallows to make them light and fluffier, whisk them to stiff peaks now and set aside. If you have a stand mixer, you can whisk them at the same time you whisk the gelatine in step 6 (unfortunately I don't so there is some juggling!)
Set a bowl of cold water next to your pan of sugar syrup. Dip your spatula into the hot syrup then drip the syrup into the water. If the blob of syrup is firm yet pliable, it's ready. Too soft: If it's still liquid or really soft, squidgy and can be completely flattened it needs longer. Too hard: If it forms little sugar marbles, it's gone too far (you can recover it by adding a little warm water to the syrup and reboiling). Remove the sugar syrup from the heat. Whisk the set gelatin with electric beaters and gradually pour the sugar syrup down the side of the bowl (this cools it and stops the gelatin from melting). Pour it in slowly and the mixture will gradually become thick, creamy and white. Once all the syrup has been added, keep whisking and it will continue to grow in volume and become glossy and smooth. Don't whisk if the mixture is cold as it will become much harder to handle!
Grease a spatula with butter or coconut oil.
Fold in the salt, vanilla, egg whites and gingerbread flavouring. (If the marshmallow mixture feels stiff and firm, quickly use the electric beaters until it's smooth.)
Pour it all into the greased dish, sprinkle in a little icing sugar, cover with a tea towel and leave to rest for at least 6 hours.
In a bowl, combine the dusting ingredients of icing sugar, cornflour and the sprinkle of cinnamon.
Slice the marshmallows into cubes and turn them out onto a plate or board covered in the marshmallow dusting.