Preheat oven to 200C (180C fan forced).

Place pastry rectangles on a lined baking tray.

Bake until golden, approximately 5 to 7 minutes, checking after 4 minutes to prevent burning.

In a saucepan, combine caster sugar and cornflour.

Gradually add milk while whisking continuously to avoid lumps.

Bring the mixture to a boil over medium heat.

Remove from heat and whisk in vanilla extract, egg yolks, and butter until smooth.

Cover the surface of the custard with cling wrap to prevent a skin from forming while cooling.

Allow the custard to cool completely.

Once cold, gently stir in whipped cream.

Split the baked pastry rectangles in half lengthwise.

Spoon the custard filling onto the flat side of one pastry half.

Top with the other pastry half, rounded side up.

Prepare passionfruit icing by mixing passion fruit seeds and juice with icing sugar.

Spoon the passionfruit icing over the vanilla slices or dust with icing sugar.

Serve immediately. If not serving immediately, assemble just before serving to maintain pastry crispness. Store pastry in an airtight container to prevent softening.