Cream butter and sugar until light and pale, about 5 minutes. Add egg yolk and beat into butter. Add flour and lemon zest and mix well.
Wrap dough in cling film and chill in the fridge for at least 1 hour until firm. Preheat the oven to 180°C and line an oven tray with baking paper.
Roll the dough, between two sheets of baking paper, out to a thickness of about 1 cm.
Cut out your shapes using your red puppy bikkie cutter. Use a fish slice to carefully transfer the cut out biscuits to the prepared oven tray. If you find the dough breaks easily, just pop it back in the fridge until it is firm again.
Bake in the oven for 12 minutes, or until golden on the edges. Leave to cool slightly before carefully transferring, using a fish slice, to a wire rack to completely cool before icing.