Preheat oven to 180°C and line two baking trays with baking paper.
Use electric mixer to beat butter and sugar until pale and creamy. Add milk and vanilla essence and beat until combined. Add sifted flour, custard powder and mix to form a soft dough.
Roll a teaspoon of mixture into a ball and place 3cm apart on baking tray. Use your finger to make an indentation in the centre of each ball and fill the hole with ¼ or ½ teaspoon of jam as desired. Then bake for 15 minutes.