Place butter and caster sugar into a large mixing bowl and whisk until light, creamy and fluffy.
Add an egg and vanilla extract into the mixing bowl and whisk together until combined.
Add flour, baking powder, lemon juice and lemon zest into the mixing bowl and whisk until a dough has formed.
Scoop a generous teaspoon amount of dough, place it onto the palm of your hand and roll the dough into a ball. Place on a baking tray, leave space between each ball as they will spread in the oven.
Bake in the oven for 12 to 15 minutes and allow the cookies to cool down once baked.
Mix icing sugar and lemon juice together in a small mixing bowl. Add a tablespoon of lemon juice and mix at a time. Add more if necessary.
Pour the icing into a piping bag and drizzle it over the cookies. You could use a teaspoon to drizzle the icing over the biscuits.