In small saucepan, combine 1/4 cup sugar, brown sugar, salt, milk, 2 tablespoons margarine and 1/2 cup chocolate chips. Bring to a boil over medium heat, stirring constantly. Reduce heat; simmer 3 minutes. Remove from heat. Stir in powdered sugar and 1/2 teaspoon vanilla; beat until smooth. Frost cooled brownies. Cut into bars. Store in refrigerator.