Put the flour in a bowl. Add the butter and rub in using your fingertips until the mixture looks like fine breadcrumbs. Stir in the sugar.
Mix in the egg yolk and add cold water a little at a time, cutting it in with a knife until the mixture comes together. Form the dough into a ball, wrap in clingfilm and refrigerate for 30 minutes.
Preheat the oven to 200°C/fan 180°C/gas mark 6. Sprinkle your work surface and rolling pin with a little flour and roll the pastry out to ¼cm thick. Cut out 24 small circles and use to line 2 x 12-hole mini tart trays. Prick the base of each tart once with a fork.
Bake for 5 minutes. Remove from oven and fill the cases with the berries. Top each with about 1 tsp jam and bake for another 10 minutes. Leave the tarts to cool in the tray for a few minutes, then use a palette knife to lift out of the tins. Leave to cool on a wire rack.