Rub half the lemon zest into the caster sugar in a bowl using your fingertips until the mixture looks slightly damp. Set 2 tbsp of the lemon sugar aside, then stir the ground almonds into the rest until combined.

Pour the egg whites into a large, clean bowl and whisk for 3-5 mins until stiff peaks form. Gradually whisk in the 2 tbsp of reserved lemon sugar to help stabilise the egg.

Mix half the beaten egg whites and the almond liqueur (if using) into the dry ingredients until you have a smooth paste, then fold in the remaining egg whites until just incorporated. Chill for 1 hr.

Heat the oven to 190C/170C fan/gas 5 and line two large baking trays with baking parchment. Roll tablespoons of the chilled dough into 15 even-sized balls.

Fold the remaining lemon zest into the icing sugar, then roll the balls in this and place 2cm apart on the prepared trays. Dust with a little extra plain icing sugar, then bake for 13-15 mins until lightly golden but still soft. Leave to cool on the trays for 10 mins, then transfer to a wire rack and leave to cool completely. Will keep in an airtight container for up to 10 days. Dust with more icing sugar just before serving.