Preheat oven to 160 C. Grease a 23x33cm (9x13 in) baking tin.
In a large bowl, mix together 200g caster sugar and 125g butter until smooth. Beat in the eggs one at a time, then stir in the vanilla and chocolate sauce. Combine the salt and flour; mix into the batter just until blended. Spread the batter evenly in the prepared tin.
Bake for 25 minutes in the preheated oven, until the brownies begin to pull away from the sides of the tin. Let cool.
In a medium bowl, mix together 125g 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.