Butter an 8” square pan. Put sugar, salt, evaporated milk, and marshmallows into a heavy 2 quart sauce pan. Stir over medium heat until mixture boils. Reduce heat and continue boiling for 5 minutes. Remove from heat. Add semisweet chocolate chips and vanilla. Stir until chocolate is melted. Pour into buttered pan; cool. Cut into squares.
Makes 1 1/2 lbs.