Mix together the almonds, salt, baking powder, lemon zest and almond essence. Gradually add the eggs one at a time and form a ball shaped dough. Add the icing sugar and mix well. Wrap in cling film and refrigerate overnight. Once the mixture has set, form into small balls and place onto non-stick baking sheets, slightly apart from each other. Flatten with the back of a spoon and bake in the oven at 180`c for 10 minutes.