Line a 23cm x 23cm square baking dish with cling film and brush with sunflower oil.
Place the caster sugar, 175ml of water and the golden syrup in a pan. Place on a medium heat and let the sugar dissolve. Whilst the sugar syrup is heating dissolve the gelatine. Place a heat proof bowl with 125ml of water and the gelatine over a pan of simmering water. Let the gelatine dissolve fully in the water, and keep warm. As the sugar in the sugar syrup dissolves, allow the mix to heat and bubble to 120 C. Then remove from the heat and pour in the melted gelatine water, whisking it in.
In a separate bowl whisk the egg whites to soft peaks. Once it has reached soft peaks continue to whisk whilst slowly pouring the sugar syrup in a steady stream. Once all the syrup is added continue to whisk on high for approximately 10 minutes until the mixture has noticeably thickened.
Dust the bottom and sides of the baking dish with half the icing sugar through a sieve. Pour the marshmallow mixture into the prepared dish and smooth the top with a palette knife.
Let the marshmallow sit at room temperature for a minimum of 6 hours to allow it to set.
Once set, sieve the remaining icing sugar on a surface and tip the marshmallow onto it, peeling back the cling film. Slice into 25 squares and dust with more icing sugar if desired.
