• Preheat the oven to 165C (fan-forced) and line 2 baking trays with baking paper.
• Using a stand mixer or electric beaters, cream the butter, sugar and vanilla until light and fluffy.
• Meanwhile, sift together the flour, baking powder and custard powder. 
• Add sifted dry ingredients to the creamed butter and sugar and mix gently at low speed to combine.
• For even-sized biscuits, weigh dough into 20g portions. Roll each portion into a ball and place it on the baking tray, leaving a few centimetres between each. Use a lightly floured fork to flatten each ball gently.
• Bake for 12 to 15 minutes or until firm and the biscuit can be moved easily on the tray when pushed gently. Another way to check is to carefully pick up one of the biscuits to check the bottom side; if it is lightly golden and the biscuit is firm they are ready. 
• Remove the trays from the oven, cool on the tray for 5 minutes, then transfer to a cooling rack to cool completely.

Icing

• Meanwhile, prepare the icing - combine the butter, icing sugar, and orange zest or passionfruit pulp and mix together until light and creamy. As this is a small quantity, you can do this by hand with a small bowl and a spoon. Add a little orange juice if the mixture is too stiff.
• Pair up the biscuits finding 2 halves that match one another, add a dollop of icing and sandwich the biscuits together using just enough pressure to coax the icing towards the edges. Note: if you'd like a more professional result, transfer the icing to a piping bag fitted with a plain or fluted piping tip, then pipe a generous blob of icing onto the biscuit, then sandwich the biscuits together. Allow to sit on a tray until the icing firms up, then transfer to an airtight container.