Place the sugar in a heavy saucepan. Cook over medium-high heat, stirring just until the sugar has melted. Continue cooking, swirling the pan often, until the sugar is dark golden caramel. Remove the pan from the heat and carefully pour in the cream. Return the pan to the heat and simmer, stirring, until the caramel has dissolved.
Remove the pan from the heat and, while hot, stir in the chocolate, peanut butter, salt, and vanilla. Let stand for about 5 minutes or until the chocolate and peanut butter have dissolved. Transfer to a bowl and cool to room temperature. Cover the bowl with plastic wrap and chill until firm, about 2 hours.
Use a small spoon to scoop out the truffles and form into 1-inch balls. Roll the truffles in the peanuts and transfer to a tray covered with waxed paper. Chill until firm, about 1 hour. Store in an airtight container for up to 2 weeks.
Recipe courtesy of Sara Moulton, Sara Moulton Cooks at Home, Broadway Books, 2002