In a double boiler, whisk together the milk, the sugar and the salt. Heat the milk over medium heat till hot but not boiling. Whisking frequently. Add the chocolate pieces and stir gently till the chocolate melts into the milk. Remove from heat and continue stirring.
Use a fork to whip a little the yogurt, pour it into the chocolate mixture and stirring.
Divide the mixture in 8 bowls. Put them on the fridge for at least 2 hours and serve.