Put down about an 18-inch length of wax paper if you are going to ladle the candies. Or grease a 9x9 pan if you are going to pour the candy into a pan.
In a 3-quart sauce pan, over medium-high heat, melt margarine, sugar and milk. Bring to soft boil stage (240). This takes awhile.
Remove from heat, add chips, vanilla and salt. Stir until chips are melted.
Ladle onto waxed paper. Top with pecan half, if desired. This sets up fast, so act quickly. Or pour into greased pan. Score. Top with pecan half. When firm, cut into squares. This is easier, but I like the way the ladled candies look better.
I have made these with butter, but they don't do as well.