Beat butter until pale and fluffy, then stir through sugar, egg yolk and lemon rind. Sift together cinnamon and flour, stir into mixture and gently knead until smooth.
Preheat oven to 180C. Roll 2 tsp of dough into balls and place on baking paper-lined oven trays and bake for 12-15 minutes or until golden. Cool on a wire rack and dust heavily with icing sugar. Biscuits will keep in an airtight container for 3 days.