Lightly oil and line the base and sides of a 20x30cm tin with non-stick baking parchment.
Grind the dried strawberries to a powder in a spice blender, or with a stick blender, and reserve.
In a large bowl, mix together the icing sugar, cornflour and ½ tbsp of the powdered strawberry, then sieve half this mixture on to the base of the lined tin.
In a small bowl, soften the gelatine leaves in cold water for about 10 minutes. Reserve 2 tbsp of the caster sugar and heat the remainder in a medium-sized pan with the golden syrup and 150ml cold water, stirring until the sugar has dissolved. Bring the sugar liquid to the boil and continue until it reaches the soft-ball stage (116°C on a sugar thermometer). Don’t stir while the liquid is boiling.
While the sugar is boiling, whisk up the egg whites with the reserved 2 tbsp caster sugar in a stand mixer until soft peaks form.
Remove the hot syrupy liquid from the heat when it reaches the correct temperature and leave it to cool for 2 minutes.
Remove the gelatine leaves from the water and pat them dry on a clean tea towel.
Add the gelatine to the hot syrup, stirring gently to dissolve it. The mixture will bubble up slightly.
With the mixer on slow, pour the hot syrup on to the beaten egg whites. Once it’s all been added, turn the mixer to high and beat until the marshmallow is thick and doubled in size (this can take up to 5 minutes). When nearly there beat in the remaining strawberry powder and vanilla powder.
Pour the marshmallow into the prepared tin and leave it to cool and set, preferably overnight.
Once set, use your fingers to ease the marshmallow away from the sides of the tin, then turn it out onto a chopping board dusted with a little of the reserved icing sugar. Cut the marshmallow widthways into strips about 3-4cm across and then cut into squares. Dust in the remaining icing sugar. Serve, or bag and give as gifts.