Preheat the oven to 200C/400F/Gas 6.
Rub together the butter or margarine with the flour in a bowl until it resembles breadcrumbs. Stir in a few tablespoons of water to form a dough.
Roll out on a lightly floured surface. Use a pastry cutter to cut out about 15 circular shapes.
Put each pastry case into an individual cupcake tray. Add a teaspoon of jam to each case â€“ not too much or it will ooze everywhere.
If there is any leftover pastry, re-roll it and cut smaller shapes, either circles or stars, and put over the jam to act as the lids.
Put the tray of tarts into the oven and cook for about 30 minutes, or until the pastry has gone pale brown (check the tarts after 20 minutes).