Since custard powder is not readily available in the States, and I was desperate to make these, I decided to try a substitute. I used cornstarch (cornflour) in place of the custard powder and added an extra 1/2 tsp of vanilla. It worked wonderfully!
I've heard you can find custard powder in some Asian food stores but haven't looked myself...
7 oz butter
1/3 cup icing/confectioners sugar
1 1/2 cups self-raising flour
1/2 cup custard powder (or cornstarch + 1/2 tsp vanilla)
1 1/2 tsp vanilla
3 Tbsp milk
Cream together the butter and the icing sugar. Add the flour, cornstarch and vanilla and beat well.
Add the milk and mix firmly until dough forms a large ball.
Roll into small balls and place on ungreased cookie sheet. Press each one down with a fork and bake on 300 for 15 minutes or until lightly browned (Be Careful! It is very easy to burn the bottoms of these - be sure to check them continually) Remove from cookie sheets and allow to cool on wire racks.
Makes 35-40 cookies.