Grease and line a 21cm x 31.5cm x 6cm-deep slab pan. Combine sugar, glucose and 250ml water in a small pan over medium heat. Cook, stirring constantly, until sugar dissolves. Bring to the boil and cook for 5 minutes or until mixture reaches 116°C on a sugar thermometer. Transfer to a jug. Cool for 10 minutes.
Whisk gelatine with 185ml boiling water until gelatine dissolves. Stir into sugar syrup.
Using an electric mixer, whisk egg whites to soft peaks. Add vanilla and slowly add sugar syrup. Whisk for 5 minutes or until cool.
Working quickly, transfer ½ cup marshmallow mixture to a small bowl and tint with food colouring.
Spoon remaining marshmallow mixture into prepared pan. Drop teaspoonfuls of tinted mixture over the top. Using a skewer, swirl colour through top of marshmallow to decorate. Stand for 3 hours or until set.
Using an oiled knife, cut into 32 squares. Dust sides of marshmallows in icing sugar, leaving top uncoated. Store for up to 1 week.
Photography by Nicholas Watt