1. Combine sugar, evaporated milk, marshmallows, and salt into a boiler.
2. Cook, stirring frequently, until marshmallows and sugar melt.
3. When bubbles cover the entire surface, boil and stir 5 minutes over medium heat.
4. Remove from heat. Beat in butter, chocolate, and vanilla until chocolate is fully melted.
5. Spread into a buttered 9" pan.
6. Chill until hard. Keep refrigerated until ready to use.
