1.Combine sugar and the water in small saucepan; stir over heat, without boiling, until sugar dissolves. Bring to the boil; boil, uncovered, without stirring, until golden brown. Add cream; stir over low heat until toffee pieces melt. Remove from heat; stir in dark chocolate and half of the salt until smooth. Refrigerate mixture overnight.
2.Working with a quarter of the chocolate mixture at a time (keep remainder refrigerated), roll rounded teaspoons of mixture into balls; place on foil-lined tray. Freeze until firm.
3.Working quickly, using two forks, dip truffles in milk chocolate. Return truffles to tray; sprinkle with remaining salt. Refrigerate truffles until firm.