Line the base and sides of a 25cm x 35cm (10in x 14in) baking tin with nonstick baking paper and dust with icing sugar. Put the gelatine in a large mixing bowl and cover with 250ml (8fl oz) cold water.
Put the golden syrup and caster sugar in a pan with 250ml (8fl oz) cold water. Bring to the boil, stirring to dissolve the sugar. Bubble for 15-20 minutes until it reaches the hard-ball stage (a little dropped into a glass of water will form a ball) or 122°C (250°F) on a jam or sugar thermometer.
Carefully pour the hot sugary syrup into the bowl containing the gelatine and stir until it has dissolved. Whisk for about 15 minutes until the marshmallow mixture is very thick and creamy white, and doubled in volume. Add the vanilla extract just before the end.
Pour the marshmallow mixture into the prepared tin, level the top with the back of a spoon and chill uncovered for 12 hours or overnight, until set firm.
Use a little more of the icing sugar to dust a work surface and turn the marshmallow out onto it, removing the baking paper. Warm a snowflake cutter (or any other shaped cutter) in hot water and use to stamp out the shapes. Dip the marshmallow shapes in a little more icing sugar and arrange on a wire rack. Leave them to dry, tossing twice more in icing sugar until the surface feels papery.
Store the marshmallows in an airtight container, layered up with nonstick baking paper and dusted with a little more icing sugar.
Store for up to 2 weeks in an airtight container.
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