Preheat oven to 150°C (130°C fan). Grease two oven trays; line trays with baking paper.
Beat butter and caster sugar in small bowl of an electric mixer until just combined; gradually beat in the combined sifted flours, icing sugar and xanthan gum. Stir in passionfruit pulp.
Turn dough onto a surface lightly floured with rice flour; knead until just smooth. Roll dough out until 5mm thick; using the heart cutter, cut 36 hearts from dough. Place hearts, about 2.5cm apart, on prepared trays.
Bake shortbread for 15 minutes or until browned lightly and just firm. Stand on trays for 5 minutes before transferring to wire racks to cool
Meanwhile, make passionfruit glaze; stir ingredients in a small bowl until smooth.
Spread tops of hearts with passionfruit glaze. Stand on wire racks until set.
You will need a 5cm heart-shaped cutter. Store the shortbread in an airtight container in the fridge for up to 2 weeks. Xanthan gum is available from health food stores.Note