Preheat oven to 180 degrees C. Grease and flour a 23x33cm baking tray with high sides. In a small bowl mix together biscuit crumbs, sugar and melted butter. Press onto the bottom of the prepared tray.
Place condensed milk and milk melts over low heat. Stir until chocolate has melted. Add vanilla. Pour into prepared base. Refrigerate until firm.
Beat the icing sugar with 90g of butter in a medium bowl. Gradually beat in milk and peppermint essence. Spread evenly over the fudge layer and refrigerate for at least 1 hour.
Melt 80g chocolate with 50g butter over medium heat or in a microwave oven, stirring frequently until smooth. Spread over the mint layer when completely cool. Chill again to set before cutting into bars.