Line a 20cm/8in shallow square cake tin with baking parchment and lightly oil.
Place the gelatine leaves in a bowl of cold water to soak for 5 minutes.
Place the egg white in a large grease-free icing bowl and get the whisk ready for when the sugar syrup is made.
To make the syrup, place the tonic water in the saucepan and add the granulated sugar. Heat gently, stirring until the sugar dissolves. Once all the sugar has completely dissolved bring the sugar syrup to a boil and continue to boil without stirring until the temperature reaches 115°C/239°F.
As soon as the sugar syrup reaches the correct temperature, remove from the heat. Lift the gelatine sheets out of the water and squeeze off the excess water then add to the syrup one at time, stirring well until dissolved. Then stir in the gin.
Working quickly, whisk the egg whites until standing in stiff peaks, then with the whisk running on a low speed, very slowly whisk the hot syrup into the egg whites. Pour it in a thin and steady stream, trying to avoid hitting the beaters.
When all the syrup has been added, add the lime zest and increase the speed of the mixer and continue whisking for about 8 to 10 minutes until you have a thick glossy mixture. The mixture will fall from the whisk in a slow ribbon like stream when it is ready.
Pour into the prepared tin and leave in a cool place (not the fridge) for 4–6 hours until set and springy to the touch.
Mix together the icing sugar and cornflour and dust the top of the marshmallow with the mixture. Turn out onto a board and carefully remove the parchment paper. Dust with a little more cornflour mixture and cut into squares.
Gently toss the squares into the cornflour mixture and serve immediately or store in an airtight container in a cool place for 3 days.