Soak the gelatine leaves in cold water for about 5-10 minutes until soft. Remove and squeeze out excess liquid, adding to a bowl with lime juice and zest.
Combine granulated sugar, caster sugar, and liquid glucose in a saucepan with 100ml of water. Heat until sugar dissolves, then boil until it reaches 120°C (248°F).
While the sugar cooks, whip egg whites in a mixer until soft peaks form, about 3-5 minutes.
Carefully pour the hot sugar syrup into the egg whites with the mixer on low, then increase to high and whip until thick and glossy, about 10-12 minutes.
Add the gelatine mixture to the whipped marshmallow and whip for an additional 2-3 minutes until combined.
Line the baking pan with parchment paper. Dust with icing sugar and cornflour. Transfer the marshmallow mixture, smoothing the top, and dust with more icing sugar and cornflour.
Let the marshmallows set at room temperature for at least 4 hours, or overnight for best results.
Once set, dust a surface with icing sugar and cornflour, turn out the marshmallow slab, cut into squares, and dust each piece to prevent sticking.