Preheat the oven to 160°C (fan-forced) and line two baking trays with baking paper.
Sift the icing sugar into a large bowl, add the butter and vanilla essence, then beat with a stand mixer or hand-held beaters until smooth and creamy.
Sift in the self-raising flour, plain flour, and custard powder, and stir well to combine.
Roll the mixture into teaspoon-sized balls, placing them 5 cm apart on the prepared trays, and press the tops lightly with a fork.
Bake for 10–12 minutes or until just lightly golden.
Leave the biscuits to cool on the trays for 10 minutes before transferring them to a wire rack to cool completely.
To make the filling, beat the butter, vanilla essence, and lemon juice until smooth and creamy, then gradually add the sifted icing sugar, beating well between additions.
Once smooth, add one drop of pink food coloring and beat until evenly mixed.
Spread about 1 teaspoon of filling onto one biscuit and sandwich with another.