Preheat oven to 180C and line 2 oven trays with baking paper. Beat butter, icing sugar, lime juice and rind, and vanilla seeds until pale and creamy (2-3 minutes). Sift flour and cornflour over the butter mixture and mix until well combined. Take a heaped tablespoonful of dough, roll into a ball and press lightly with a floured fork twice to form a cross-hatch pattern, then place on lined trays. Bake until just golden (12-15 minutes). Cool briefly on trays, then transfer to a wire rack.
Meanwhile, for passionfruit curd, whisk yolks in a heatproof bowl to combine. Combine sugar, passionfruit pulp and butter in a saucepan over low heat and stir until butter melts and sugar dissolves (5 minutes). Whisking continuously, add a third of the passionfruit mixture to yolks, then add to pan and stir continuously until thickened (3 minutes; do not boil). Add lime juice, remove from heat and pass through a coarse sieve into a container. Cover directly with plastic wrap, cool for 10 minutes, then refrigerate until chilled (1 hour).
For lime butter, beat ingredients in an electric mixer until pale and fluffy (2-3 minutes).
Top half the biscuits with lime butter, then passionfruit curd (there may be a little left over), sandwich with remaining biscuits and refrigerate until firm (30 minutes). Filled yo-yos will keep refrigerated in an airtight container for 2 days.