Take the butter out of the fridge in good time so that it becomes soft enough to easily press the tip of your finger in and leave an indentation.
Grate the zest from the lemon.
Add the lemon zest to the sugar and mix in thoroughly.
Cut the softened butter into chunks and place onto a worksurface. Add the sugar and lemon zest mix to the butter pieces. Next to the sugar, butter and lemon zest pile, pour the flour into a mound.
Using your fingertips, work the sugar and butter together until thoroughly mixed.
Gradually work the flour into the sugar and butter mixture, using your fingers, until all ingredients are thoroughly mixed together.
Place the dough onto greaseproof paper, wrap the greaseproof paper around it and shape into a cylinder shape. Place in a fridge and chill for at least 2 to 3 hours.
Cut the chilled dough into slices about 0.5cm thick. Start heating the oven. 160 degrees C, 325 degrees F, Gas Mark 4.
Put a sheet of greaseproof paper onto a baking tray and place the slices onto the paper, spaced apart. The dough slices will spread slightly during cooking.
Bake for about 15 to 20 minutes (160 degrees C, 325 degrees F, Gas Mark 4), until the biscuits are very slightly bronzed at the edges.
Whilst the biscuits are still hot, sprinkle with a little caster sugar for decoration.