Using an electric mixer, beat butter and sugar until light and fluffy. Add orange rind and vanilla and beat until just combined. Sift flours and ½ tsp salt over butter mixture and, using a wooden spoon, stir to form a soft dough.
Form level ½ tablespoons of mixture into balls, then place 5cm apart on baking paper-lined oven trays, flatten slightly to 3.5cm rounds, then, using a floured fork, press tines gently into dough rounds, to create grooves. Bake at 180C for 12 minutes or until lightly coloured. Stand biscuits on trays for 5 minutes, before transferring to wire racks to cool completely.
For passionfruit icing, beat sugar and butter until light and fluffy, add passionfruit and beat until combined. Sandwich biscuits together with 2 tsp of icing, then dust with icing sugar.