Prepare a cookie sheet by lining with wax paper lightly sprayed with non-stick cooking spray and set aside. Then combine the sugar, salt, water and margarine in a saucepan. Cook on medium high heat, while stirring, until it reaches light crack stage; 285 degrees on a candy thermometer. Pour out on prepared wax paper. Cool until hard. You can speed up the cooling process in the refrigerator. Melt chocolate chips in a double boiler. Pour 1/2 of the melted chocolate over the hardened toffee.Spread and cool. Turn the toffee over on another piece of sprayed wax paper and peel off the first piece. Pour the remaining chocolate (you may have to remelt)on the other side of the toffee. Cool. Break with a hammer and enjoy!