Cream butter and sugar until light and fluffy.
Mix in condensed milk and vanilla.
Sift flour and custard powder (or cornflour) together and slowly mix into the butter and sugar until all combined. If the mix is too soft to roll into balls pop the bowl in the fridge to set for 10-15 mins. Roll into balls, place on a liked tray and flatten with a fork dipped in water.
Bake in a preheated 175*C oven on a lined tray for about 10-15mins. Cool and freeze if you wish.