For the pastry, place the flour, butter and icing sugar into a food processor. Pulse briefly until the mixture resembles breadcrumbs, then add the egg yolk and one tablespoon of cold water.
Pulse again until the mixture sticks together in clumps then tip onto a work surface and gather it into a ball with your hands. Knead the pastry just two or three times to make it into a smooth ball. Wrap it in cling film and place in the refrigerator for 15 minutes.
Grease 12 x 7.5cm/3in fluted tart tins.
Roll out the pastry to a thickness of 5mm/¼in. Cut out circles with a 10cm/4in round cutter and use to line the tins, re-rolling the pastry as necessary. Place the tart cases in the fridge to chill.
Preheat the oven to 200C(180C fan)/400F/Gas 6.
Cover the base of the tartlets with baking parchment or foil and fill with a few baking beans.
Bake blind for seven minutes then remove the parchment and beans.
Return the pastry cases to the oven for another 4-5 minutes or until they are light golden-brown and completely dry. Set aside to cool while you make the filling. Reduce the oven temperature to 170C(150 fan)/325F/Gas 3.
For the filling, break the eggs into a large bowl and whisk together with a wire whisk. Add the rest of the filling ingredients and whisk again until they are all well-combined. Pour the filling mixture into a jug, then into the cooled baked pastry cases. To prevent it spilling as they go into the oven, pour in most of the filling so it almost fills the tarts, carefully sit the baking sheet and tarts on the oven shelf, then top up with the rest of the filling to completely fill the tarst.
Bake for about seven minutes, or until just set but with a slight wobble in the centre.
Leave to cool slightly then carefully ease the tartlets from their tins an place on a wire rack to cool completely.
For the decoration, melt the chocolate in a bowl set over a pan of simmering water. (Do not allow the bottom of the bowl to touch the surface of the water.)
Spoon the melted chocolate into a small disposable piping bag. Snip off the end and pipe the word ‘citron’ or a decoration of your choice on top of the tarts.