Preheat oven to 160 C. Grease a 20-23cm x30-33cm baking pan.
In a large bowl, beat together caster sugar and butter until smooth. Beat in the eggs one at a time, then stir in the vanilla and chocolate sauce.
Combine the salt and flour. Stir into the chocolate mixture just until blended. Spread into the prepared pan evenly.
Bake in preheated oven for 25 minutes until the brownies begin to pull away from the sides of the pan. Allow to cool.
In a medium bowl, mix together melted butter and icing sugar until smooth. Stir in creme de menthe liqueur.
Spread over the cooled brownies and allow to cool completely.
Combine the chocolate and remaining butter in a microwave safe dish. Heat for 1 minute in the microwave, stir, then continue to heat at 30 second intervals, stirring each time, until melted and smooth.
Spread over the top of the mint layer. Allow to cool completely before cutting into squares.