Heat the oven to 180°C.
Lightly grease a shallow muffin tin or scone tin. Cut the pastry into circles, using a glass or cookie cutter ( big enough to fit in the tin holes). Press the disks into the wholes.
Mix the custard with the egg yolk and cinnamon (or citrus zest).
If you're using fruits/dried fruits, scatter a couple of pieces into each case.
Pour the custard into the pastry cases and bake for 15-20 mins, until golden.
Transfer the tarts to a wire rack to cool.
Serve at room temperature, dusted with icing sugar, if liked.