Lightly grease and line a 17.5cm x 25cm shallow cake pan and dust base liberally with dusting mix. Combine yuzu juice and gelatine in a bowl and set aside.
Combine caster sugar and 1 cup water in a saucepan and cook over low heat, stirring, until sugar dissolves, then increase heat to medium and cook for 5-10 minutes or until syrup reaches 125°C on a sugar thermometer. Remove from heat, add yuzu mixture to syrup and stir until gelatine dissolves.
Meanwhile, using an electric mixer, whisk eggwhites and a pinch of salt until mixture forms soft peaks.
Gradually add yuzu mixture, whisking continuously on medium speed until mixture has doubled in size, then slowly decrease speed and mix until mixture is warm (about 40°C).
Pour into prepared cake pan, and, using a lightly oiled spatula, spread evenly, then dust top liberally with the dusting mix. Stand at room temperature for 3 hours or until firm.
Using a sharp knife, cut marshmallow into 2.5cm squares and roll in dusting mix to coat.
Store in an airtight container between sheets of baking paper at room temperature for up to 2 weeks.