1. Combine the cream, chocolate, and sugar in a heavy saucepan over low heat. Cook, stirring constantly, until the chocolate melts and the mixture is smooth. Add the salt and let the mixture come to a boil. Turn down the heat to very low and cook without stirring until the mixture reaches the soft-ball stage, 236°F on candy thermometer.

2. Remove the pan from the heat. Add the butter and vanilla, but do not stir. Let the mixture cool until lukewarm (110°F). Then beat the fudge with a wooden spoon until creamy. Add the nuts and mix well. Transfer the fudge to a buttered 8-inch-square baking pan. Cover and chill. When firm, cut into small squares.