Preheat the oven to 180ºC. Line two baking trays with baking paper.
Cream the butter and icing sugar until light and fluffy and then add the vanilla.
Sift the flour and custard powder together and combine with the creamed mixture to form a soft dough.
Roll teaspoonfuls of the mixture into balls, place on the prepared trays and press down with a fork.
Bake for 15–20 minutes until golden. Leave on the trays for 1–2 minutes then place on a wire rack to cool.
Mix all the filling ingredients together until light and fluffy.
When cold, sandwich in pairs with the Butter Filling.