Preheat oven to 160°C/140°C fan-forced. Line 2 baking trays with baking paper.
Using an electric mixer, beat butter, sugar and vanilla until fluffy.
Sift flour and custard powder over butter mixture. Stir with a wooden spoon until soft dough forms.
Roll level tablespoons dough into 32 balls. Place on prepared trays, leaving room for spreading.
Using a fork dipped in flour, lightly flatten each biscuit until 1cm thick.
Bake for 15 to 20 minutes or until light golden. Cool on tray for 10 minutes. Transfer to a wire rack to cool completely.
Meanwhile, make filling. Using an electric mixer, beat butter, sugar and lemon rind in a bowl until light and creamy.
Spread the flat side of 1 biscuit with 2 teaspoons filling. Sandwich with 1 biscuit. Repeat with the remaining biscuits and filling.