Preheat oven to 190 C / Gas 5. Grease a baking tray or line with baking parchment.
In a medium bowl, cream butter, vanilla and sugar. Add the egg and mix well.
Sift in flours and custard powder and stir. Roll into balls and place on prepared tray. Lightly flatten with a fork. Make a small depression with your finger or the end of wooden spoon, in the top of each ball and add a little jam.