Cream butter and sugar. Beat in egg yolk and vanilla. Stir in flour just until combined – do not overmix. Fold dough into a ball and wrap it in plastic wrap; chill dough for at least 30 minutes. Break chilled dough into nine small pieces and press each piece into a small tart mould. Trim excess dough off the top of the moulds to make the tarts neat. Pierce the bottoms of the dough with a fork and place the moulds in the freezer for 10 minutes. Bake on the lowest rack of oven preheated to 340°F for 20 minutes or until golden brown. Let tarts cool before removing them from moulds.
Fill cooled tarts with blueberry jam and top with a dollop of cream.