Preheat the oven to 180°C/gas mark 4. Line 2 baking trays with baking parchment. Beat together the butter and icing sugar until light and fluffy. Add the flour, cornflour and vanilla extract and beat until the mixture just comes together.
Using your hands, roll the mixture into 16 small balls. Flatten slightly into discs and place on baking trays then, using a lightly floured fork, press down gently on the top of each biscuit. Bake for 12-15 minutes or until just starting to brown. Cool on a wire rack.
To make the filling, beat together the butter, icing sugar and passion fruit juice until creamy. Use to sandwich the biscuit halves together.