Sift the flour and salt into a bowl and set aside. Cream together the butter and sugar in a large bowl, using a wooden spoon - or in a food mixer - until well combined and fluffy. Add the egg yolk and vanilla and mix it in well, then work in the flour. Once the mixture comes together as a dough, dust your hands with flour and wrap it in clingfilm. Chill for an hour (or up to a few days if more convenient) or freeze for up to a month. This dough can otherwise be tricky to work, especially in hot weather or a warm kitchen. When ready to bake,
Roll out the dough on a floured board to 3mm thick, using plenty of flour as it is quite sticky. (You could even roll it between 2 sheets of clingfilm if that makes it easier.) Cut out into your preferred shapes and place on 2 baking sheets lined with baking parchment.
Put the chocolate into a bowl over gently simmering water, ensuring the base of the bowl does not touch the water. Stir occasionally until melted. Cool slightly, then spoon into the piping bag. Seal the bag and snip the end. Drizzle the chocolate over the biscuits to decorate.