Mix your sugar and evaporated milk in a large saucepan.
Cook on low heat until sugar is dissolved. Stirring almost constantly. Keep an eye on this as it can burn easily.
Boil on low heat for 7 minutes, stirring almost constantly.
Remove from burner and add chocolate bits and margarine. Stir until melted.
Add your vanilla (and nuts if you want) and pour into a large pan. (I use a 9x13 pan.)
Place into the refrigerator. Cut after hardened.