1. Butter a 20cm (8 in) dish.
2. Place butter, chocolate, and vanilla in a mixing bowl.
3. In a medium saucepan over medium heat, combine sugar, milk, and marshmallows.
4. Bring to the boil, stirring frequently.
5. Reduce heat to low and cook 6 minutes more, stirring constantly.
6. Remove from heat.
7. Beat entire mixture until it thickens and loses its gloss.
8. Quickly fold in nuts and pour into prepared tin.
9. Refrigerate several hours until firm.
