Place the butter in a bowl, whisk in the sugar and salt. Add the eggs one at a time and continue to whisk until the mixture becomes lighter in colour. Add the lemon zest and flour, mix into a dough using a wooden spoon, flatten a little. Cover the biscuit dough and chill for at least 2 hrs.
Remove the dough from the fridge approx. 15 mins. prior to rolling. On a lightly floured surface or between a cut-open plastic bag, roll out the dough in batches to approx. 7 mm thick. Cut out popular shapes (e.g. heart), place the biscuits on two trays lined with baking paper. Chill the biscuits for approx. 15 mins.
Whisk the egg yolk with the water, glaze the biscuits with the egg wash. Optional: to decorate, gently drag a fork diagonally across the glazed biscuits prior to baking.
To bake each tray
Approx. 10 mins. in the centre of an oven preheated to 200°C. Remove from the oven, leave to cool on a cooling rack.