Pulse flour, butter, and icing sugar in a food processor until the mixture resembles breadcrumbs.
Add egg yolk and 15ml of cold water and pulse until the mixture clumps.
Knead the pastry into a smooth ball and chill in the refrigerator for 15 minutes.
Grease twelve 7.5cm tart tins.
Roll pastry to 5mm thickness and cut 10cm circles to line the tins.
Preheat the oven to 180°C/200°C.
Bake blind with parchment and beans for seven minutes.
Remove beans and bake for another 5 minutes until golden, then reduce oven to 150°C/170°C.
Whisk eggs, cream, sugar, and lemon together in a bowl.
Pour filling into pastry cases and bake for seven minutes until just set.
Cool tartlets on a wire rack.
Melt chocolate in a bowl over simmering water.
Pipe chocolate decorations onto the cooled tarts.