Cream together the butter, icing sugar and lemon zest until light and fluffy. Add egg yolks one at the time and mix thoroughly. Sift the flour and mix with a wooden spoon until the mixture comes together. Spoon into a piping bag with an 8mm star nozzle. Pipe roster to fill the baking tray, spacing well apart. bake for 10 min or until edges are golden. Let them cool. Divide biscuits into pairs and spread jam on the flat side and sandwich the two together and dust lightly with icing sugar.