Wash the grapes and the physalis (if using), then pat dry with kitchen paper. Snip the grapes into tiny bunches of 2–3 grapes.
Sift the icing sugar into a bowl, then gradually mix in the fruit juice to make a smooth, thick frosting.
Dip the grapes a bunch at a time into the frosting, then dip the physalis. Arrange the grapes and physalis on a wire rack set over a baking sheet. Leave in a cool place for 1 hour or until hardened.
Sprinkle the grapes and physalis with caster sugar before serving.