To make the lemon curd, blitz the lemon zest, until very fine, in a small food processor. Put the zest, sugar and butter into a heatproof bowl. Sit the bowl over a pan of gently simmering water, making sure the water isn’t touching the bowl. Stir occasionally, until the butter has melted. Add the yolks and lemon juice, whisking continually. Cook gently for about 15 minutes or until the mixture is thick. It should leave ribbons in the curd when you pull the whisk out. Leave to cool, then spoon into a piping bag with a plain 1cm nozzle.