Line 8-inch square baking pan with foil.
Combine sugar, evaporated milk, butter and salt in a medium, heavy-duty saucepan. Bring to a full rolling boil over medium heat, stirring constantly.
Boil, stirring constantly, for 4 to 5 minutes. Remove from heat.
Stir in marshmallows, chocolate chips, nuts, if using and vanilla extract.
Stir vigorously for 1 minute or until marshmallows are melted. Pour into prepared baking pan; refrigerate for 2 hours or until firm. Lift from pan; remove foil. Cut into 48 pieces.