Preheat the oven to 180°C (350°F) and line a baking tray with baking paper.
In a large bowl, cream the butter and sugar until light and fluffy.
Beat in the egg and vanilla essence.
In a separate bowl, sift together the flour, custard powder, baking powder, and salt.
Gradually fold the dry ingredients into the wet mixture until a soft dough forms.
Roll small portions of dough into balls (about 2–3 cm) and flatten slightly with your fingers or a fork.
Place on the prepared baking tray, leaving space between biscuits.
Bake for 12–15 minutes or until lightly golden at the edges.
Cool on a wire rack before serving.