Soak the gelatine leaves in ice-cold water until softened.
Sift the icing sugar and corn starch together. Generously dust a deep 15cm by 25cm pan with the icing sugar mix, set aside. Keep the rest of the dusting mix for later.
Place the tonic water, glucose syrup, caster sugar and food colour in a saucepan along with a candy thermometer. Bring the syrup up to the boil until it reaches 130'C.
Meanwhile, drain the gelatine leaves, and squeeze out any excess water. Gently heat the gelatine with the gin to dissolve.
Zest the limes, and add to the gin mix.
In a mixing bowl, beat the egg white at a low speed to obtain a frothy mixture.
When the syrup reaches 130'C, continue beating the egg whites at high speed all the while pouring the hot sugar in a thin stream. Add the melted gelatine next, and continue beating the meringue until tripled in volume and nearly cooled down to room temperature.
Pour the meringue into the prepared tray, dust with more icing sugar mix, and use your hands to press down the marshmallow until it is of even thickness.
Chill the marshmallow in the refrigerator for at least 30 minutes.
When the marshmallow is set, cut into squares, and roll in the icing sugar mix. Shake off any excess dust before serving.
«If you do not want alcohol in the marshmallows, use a flower water such as orange or rose instead.»