Mix the caster sugar with the butter, egg yolk, lemon zest and salt. Knead the two types of flour into the sugar mixture until a large ball of dough is formed. Cover the dough in cling film and let it rest in the fridge for 1 hour. Sprinkle white sugar (fine) on your working space and roll out the dough until the rolls are 3.5cm round. Cut the rolls in slices which are 1cm thick and place them on a baking tray covered with baking paper. Bake the cookies in a pre-heated oven at 170ºC for approximately 15 minutes.