In 2 quart microwaveable container place the margarine and melt for 1 1/2 minutes. Add the box of brown sugar, dash of salt, light corn syrup and sweetened condensed milk. Cook on high for 15-16 minutes stirring every 5 minutes until firm ball stage (Aunt Nelda only had to cook hers for 12 minutes in her microwave - depending on wattage of microwave) Firm ball stage is the key to this. Add 1 teaspoon vanilla. Pour into buttered 9x12 pan. Cool overnight. Then can be wrapped individually in wax paper.