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 12-15mins. Cool and eat or freeze if you wish.
Mix in chopped nuts, chocolate chips or any other additives you'd like.
I find that these make a cakey biscuit for me which I don't like to crisp them up a little I press lightly with the fork again at the 8 minute mark and then let finish baking.