Add the chocolate to the milk. Heat in the top part of a double boiler over hot water. Beat until the chocolate is blended.
Combine the sugar, flour and salt. Add gradually to the chocolate mixture and cook, stirring constantly, until thickened. Continue to cook for ten minutes longer, stirring occasionally.
Stir a little of the hot mixture into the egg yolk, then stir the yolk into the hot sauce and cook for two minutes longer. Remove from the heat and add the vanilla. Cool. Before serving, fold in the whipped cream.