Preheat the oven to 200C/400F/Gas mark 6
Cream together the butter, icing sugar and lemon essence. Stir in flour and salt. Cover and refrigerate for two hours. Shape into 1 inch balls. Place about 1 inch apart on ungreased baking tray, flatten slightly. Bake until edges are light brown, 8-10 minutes. Remove from baking tray to cool. Sandwich the Cookies together with lemon butter filling. Roll in icing sugar.
For the filling. Mix the sugar, cornstarch and salt in a saucepan. Stir in remaining ingredients. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir for 1 minute, cool.
