Beat butter, 40gm icing sugar, lemon rind and aniseed in an electric mixer until pale and creamy (7-8 minutes). Scrape down sides of bowl, add egg yolk, sherry and lemon juice and beat to combine. Sieve in flour, stir to combine then turn onto a lightly floured surface, shape into a disc, cover in plastic wrap and refrigerate until firm (1 hour).
Preheat oven to 150C. Roll dough on a lightly floured surface to 1cm thick, cut out rounds with a 4cm-diameter cutter and place on baking paper-lined oven trays. Re-roll scraps and repeat. Bake until light brown (15-20 minutes), cool slightly, then sieve remaining sugar over biscuits. Cool completely and roll in extra sugar. Store in an airtight container, covered liberally with sugar, for up to a week.