Beat 1 1/2 sticks margarine and sugar together. Add eggs, salt and vanilla. Mix in flour and cocoa. Beat on medium speed for 3 minutes. Pour into a greased 9x13 pan. Bake at 350°F for 30 minutes (top of brownies will still appear wet). Cool completely on a wire rack. Combine confectioners’ sugar, 1/2 cup butter (softened), water, mint extract and food coloring. Beat until creamy. Spread over cooled brownies. Refrigerate until set. For topping, melt chocolate chips and remaining stick of margarine over low heat in a small saucepan. Let cool for 30 minutes or until luke-warm, stirring occasionally. Spread over filling. Chill before cutting. Store in refrigerator.