Heat the oven to 180C/fan 160C/gas 4. Whizz the flour and butter to breadcrumbs, add the sugar, lemon zest and egg yolk and process to a dough. Roll the dough to 5mm thick and cut out biscuits. Put on a buttered baking sheet and cook for 15-20 minutes. Cool.
Sift the icing sugar into a bowl and mix with enough of the lemon juice to make a runny icing. Drizzle the lemon icing over the biscuits.